ERT: Emacs Lisp Regression Testing

ERT is a tool for automated testing in Emacs Lisp. Its main features are facilities for defining tests, running them and reporting the results, and for debugging test failures interactively.

ERT is similar to tools for other environments such as JUnit, but has unique features that take advantage of the dynamic and interactive nature of Emacs. Despite its name, it works well both for test-driven development (see http://en.wikipedia.org/wiki/Test-driven_development) and for traditional software development methods.

Introduction A simple example of an ERT test.
How to Run Tests Run tests in Emacs or from the command line.
How to Write Tests How to add tests to your Emacs Lisp code.
How to Debug Tests What to do if a test fails.
Extending ERT ERT is extensible in several ways.
Other Testing Concepts Features not in ERT.
GNU Free Documentation License The license for this documentation.

Detailed Node Listing

How to Run Tests

How to Write Tests
The should Macro A powerful way to express assertions.
Expected Failures Tests for known bugs.
Tests and Their Environment Don't depend on customizations; no side effects.
Useful Techniques Some examples.
How to Debug Tests
Understanding Explanations How ERT gives details on why an assertion failed.
Interactive Debugging Tools available in the ERT results buffer.
Extending ERT
Defining Explanation Functions Teach ERT about more predicates.
Low-Level Functions for Working with Tests Use ERT's data for your purposes.
Other Testing Concepts
Mocks and Stubs Stubbing out code that is irrelevant to the test.
Fixtures and Test Suites How ERT differs from tools for other languages.
Appendix
GNU Free Documentation License The license for this documentation.

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1 Introduction

ERT allows you to define tests in addition to functions, macros, variables, and the other usual Lisp constructs. Tests are simply Lisp code: code that invokes other code and checks whether it behaves as expected.

ERT keeps track of the tests that are defined and provides convenient commands to run them to verify whether the definitions that are currently loaded in Emacs pass the tests.

Some Lisp files have comments like the following (adapted from the package pp.el):

     ;; (pp-to-string '(quote quote))          ; expected: "'quote"
     ;; (pp-to-string '((quote a) (quote b)))  ; expected: "('a 'b)\n"
     ;; (pp-to-string '('a 'b))                ; same as above

The code contained in these comments can be evaluated from time to time to compare the output with the expected output. ERT formalizes this and introduces a common convention, which simplifies Emacs development, since programmers no longer have to manually find and evaluate such comments.

An ERT test definition equivalent to the above comments is this:

     (ert-deftest pp-test-quote ()
       "Tests the rendering of `quote' symbols in `pp-to-string'."
       (should (equal (pp-to-string '(quote quote)) "'quote"))
       (should (equal (pp-to-string '((quote a) (quote b))) "('a 'b)\n"))
       (should (equal (pp-to-string '('a 'b)) "('a 'b)\n")))

If you know defun, the syntax of ert-deftest should look familiar: This example defines a test named pp-test-quote that will pass if the three calls to equal all return true (non-nil).

should is a macro with the same meaning as cl-assert but better error reporting. See The should Macro.

Each test should have a name that describes what functionality it tests. Test names can be chosen arbitrarily—they are in a namespace separate from functions and variables—but should follow the usual Emacs Lisp convention of having a prefix that indicates which package they belong to. Test names are displayed by ERT when reporting failures and can be used when selecting which tests to run.

The empty parentheses () in the first line don't currently have any meaning and are reserved for future extension. They also make the syntax of ert-deftest more similar to that of defun.

The docstring describes what feature this test tests. When running tests interactively, the first line of the docstring is displayed for tests that fail, so it is good if the first line makes sense on its own.

The body of a test can be arbitrary Lisp code. It should have as few side effects as possible; each test should be written to clean up after itself, leaving Emacs in the same state as it was before the test. Tests should clean up even if they fail. See Tests and Their Environment.

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2 How to Run Tests

You can run tests either in the Emacs you are working in, or on the command line in a separate Emacs process in batch mode (i.e., with no user interface). The former mode is convenient during interactive development, the latter is useful to make sure that tests pass independently of your customizations; and it allows you to invoke tests from makefiles, and to write scripts that run tests in several different Emacs versions.

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2.1 Running Tests Interactively

You can run the tests that are currently defined in your Emacs with the command M-x ert RET t RET. (For an explanation of the t argument, see Test Selectors.) ERT will pop up a new buffer, the ERT results buffer, showing the results of the tests run. It looks like this:

     Selector: t
     Passed: 31
     Failed: 2 (2 unexpected)
     Total:  33/33
     
     Started at:   2008-09-11 08:39:25-0700
     Finished.
     Finished at:  2008-09-11 08:39:27-0700
     
     FF...............................
     
     F addition-test
         (ert-test-failed
          ((should
            (=
             (+ 1 2)
             4))
           :form
           (= 3 4)
           :value nil))
     
     F list-test
         (ert-test-failed
          ((should
            (equal
             (list 'a 'b 'c)
             '(a b d)))
           :form
           (equal
            (a b c)
            (a b d))
           :value nil :explanation
           (list-elt 2
                     (different-atoms c d))))

At the top, there is a summary of the results: we ran all tests defined in the current Emacs (Selector: t), 31 of them passed, and 2 failed unexpectedly. See Expected Failures, for an explanation of the term unexpected in this context.

The line of dots and Fs is a progress bar where each character represents one test; it fills while the tests are running. A dot means that the test passed, an F means that it failed. Below the progress bar, ERT shows details about each test that had an unexpected result. In the example above, there are two failures, both due to failed should forms. See Understanding Explanations, for more details.

In the ERT results buffer, TAB and S-TAB cycle between buttons. Each name of a function or macro in this buffer is a button; moving point to it and typing RET jumps to its definition.

Pressing r re-runs the test near point on its own. Pressing d re-runs it with the debugger enabled. . jumps to the definition of the test near point (RET has the same effect if point is on the name of the test). On a failed test, b shows the backtrace of the failure.

l shows the list of should forms executed in the test. If any messages were generated (with the Lisp function message) in a test or any of the code that it invoked, m will show them.

By default, long expressions in the failure details are abbreviated using print-length and print-level. Pressing L while point is on a test failure will increase the limits to show more of the expression.

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2.2 Running Tests in Batch Mode

ERT supports automated invocations from the command line or from scripts or makefiles. There are two functions for this purpose, ert-run-tests-batch and ert-run-tests-batch-and-exit. They can be used like this:

     emacs -batch -l ert -l my-tests.el -f ert-run-tests-batch-and-exit

This command will start up Emacs in batch mode, load ERT, load my-tests.el, and run all tests defined in it. It will exit with a zero exit status if all tests passed, or nonzero if any tests failed or if anything else went wrong. It will also print progress messages and error diagnostics to standard output.

If ERT is not part of your Emacs distribution, you may need to use -L /path/to/ert/ so that Emacs can find it. You may need additional -L flags to ensure that my-tests.el and all the files that it requires are on your load-path.

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2.3 Test Selectors

Functions like ert accept a test selector, a Lisp expression specifying a set of tests. Test selector syntax is similar to Common Lisp's type specifier syntax:

Selectors that are frequently useful when selecting tests to run include t to run all tests that are currently defined in Emacs, "^foo-" to run all tests in package foo (this assumes that package foo uses the prefix foo- for its test names), result-based selectors such as (or :new :unexpected) to run all tests that have either not run yet or that had an unexpected result in the last run, and tag-based selectors such as (not (tag :causes-redisplay)) to run all tests that are not tagged :causes-redisplay.

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3 How to Write Tests

ERT lets you define tests in the same way you define functions. You can type ert-deftest forms in a buffer and evaluate them there with eval-defun or compile-defun, or you can save the file and load it, optionally byte-compiling it first.

Just like find-function is only able to find where a function was defined if the function was loaded from a file, ERT is only able to find where a test was defined if the test was loaded from a file.

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3.1 The should Macro

Test bodies can include arbitrary code; but to be useful, they need to check whether the code being tested (or code under test) does what it is supposed to do. The macro should is similar to cl-assert from the cl package (see Assertions), but analyzes its argument form and records information that ERT can display to help debugging.

This test definition

     (ert-deftest addition-test ()
       (should (= (+ 1 2) 4)))

will produce this output when run via M-x ert:

     F addition-test
         (ert-test-failed
          ((should
            (=
             (+ 1 2)
             4))
           :form
           (= 3 4)
           :value nil))

In this example, should recorded the fact that (= (+ 1 2) 4) reduced to (= 3 4) before it reduced to nil. When debugging why the test failed, it helps to know that the function + returned 3 here. ERT records the return value for any predicate called directly within should.

In addition to should, ERT provides should-not, which checks that the predicate returns nil, and should-error, which checks that the form called within it signals an error. An example use of should-error:

     (ert-deftest test-divide-by-zero ()
       (should-error (/ 1 0)
                     :type 'arith-error))

This checks that dividing one by zero signals an error of type arith-error. The :type argument to should-error is optional; if absent, any type of error is accepted. should-error returns an error description of the error that was signaled, to allow additional checks to be made. The error description has the format (ERROR-SYMBOL . DATA).

There is no should-not-error macro since tests that signal an error fail anyway, so should-not-error is effectively the default.

See Understanding Explanations, for more details on what should reports.

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3.2 Expected Failures

Some bugs are complicated to fix, or not very important, and are left as known bugs. If there is a test case that triggers the bug and fails, ERT will alert you of this failure every time you run all tests. For known bugs, this alert is a distraction. The way to suppress it is to add :expected-result :failed to the test definition:

     (ert-deftest future-bug ()
       "Test `time-forward' with negative arguments.
     Since this functionality isn't implemented, the test is known to fail."
       :expected-result :failed
       (time-forward -1))

ERT will still display a small f in the progress bar as a reminder that there is a known bug, and will count the test as failed, but it will be quiet about it otherwise.

An alternative to marking the test as a known failure this way is to delete the test. This is a good idea if there is no intent to fix it, i.e., if the behavior that was formerly considered a bug has become an accepted feature.

In general, however, it can be useful to keep tests that are known to fail. If someone wants to fix the bug, they will have a very good starting point: an automated test case that reproduces the bug. This makes it much easier to fix the bug, demonstrate that it is fixed, and prevent future regressions.

ERT displays the same kind of alerts for tests that pass unexpectedly as it displays for unexpected failures. This way, if you make code changes that happen to fix a bug that you weren't aware of, you will know to remove the :expected-result clause of that test and close the corresponding bug report, if any.

Since :expected-result evaluates its argument when the test is loaded, tests can be marked as known failures only on certain Emacs versions, specific architectures, etc.:

     (ert-deftest foo ()
       "A test that is expected to fail on Emacs 23 but succeed elsewhere."
       :expected-result (if (string-match "GNU Emacs 23[.]" (emacs-version))
                            :failed
                          :passed)
       ...)

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3.3 Tests and Their Environment

The outcome of running a test should not depend on the current state of the environment, and each test should leave its environment in the same state it found it in. In particular, a test should not depend on any Emacs customization variables or hooks, and if it has to make any changes to Emacs's state or state external to Emacs (such as the file system), it should undo these changes before it returns, regardless of whether it passed or failed.

Tests should not depend on the environment because any such dependencies can make the test brittle or lead to failures that occur only under certain circumstances and are hard to reproduce. Of course, the code under test may have settings that affect its behavior. In that case, it is best to make the test let-bind all such setting variables to set up a specific configuration for the duration of the test. The test can also set up a number of different configurations and run the code under test with each.

Tests that have side effects on their environment should restore it to its original state because any side effects that persist after the test can disrupt the workflow of the programmer running the tests. If the code under test has side effects on Emacs's current state, such as on the current buffer or window configuration, the test should create a temporary buffer for the code to manipulate (using with-temp-buffer), or save and restore the window configuration (using save-window-excursion), respectively. For aspects of the state that can not be preserved with such macros, cleanup should be performed with unwind-protect, to ensure that the cleanup occurs even if the test fails.

An exception to this are messages that the code under test prints with message and similar logging; tests should not bother restoring the *Message* buffer to its original state.

The above guidelines imply that tests should avoid calling highly customizable commands such as find-file, except, of course, if such commands are what they want to test. The exact behavior of find-file depends on many settings such as find-file-wildcards, enable-local-variables, and auto-mode-alist. It is difficult to write a meaningful test if its behavior can be affected by so many external factors. Also, find-file has side effects that are hard to predict and thus hard to undo: It may create a new buffer or reuse an existing buffer if one is already visiting the requested file; and it runs find-file-hook, which can have arbitrary side effects.

Instead, it is better to use lower-level mechanisms with simple and predictable semantics like with-temp-buffer, insert or insert-file-contents-literally, and to activate any desired mode by calling the corresponding function directly, after binding the hook variables to nil. This avoids the above problems.

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3.4 Useful Techniques when Writing Tests

Testing simple functions that have no side effects and no dependencies on their environment is easy. Such tests often look like this:

     (ert-deftest ert-test-mismatch ()
       (should (eql (ert--mismatch "" "") nil))
       (should (eql (ert--mismatch "" "a") 0))
       (should (eql (ert--mismatch "a" "a") nil))
       (should (eql (ert--mismatch "ab" "a") 1))
       (should (eql (ert--mismatch "Aa" "aA") 0))
       (should (eql (ert--mismatch '(a b c) '(a b d)) 2)))

This test calls the function ert--mismatch several times with various combinations of arguments and compares the return value to the expected return value. (Some programmers prefer (should (eql EXPECTED ACTUAL)) over the (should (eql ACTUAL EXPECTED)) shown here. ERT works either way.)

Here's a more complicated test:

     (ert-deftest ert-test-record-backtrace ()
       (let ((test (make-ert-test :body (lambda () (ert-fail "foo")))))
         (let ((result (ert-run-test test)))
           (should (ert-test-failed-p result))
           (with-temp-buffer
             (ert--print-backtrace (ert-test-failed-backtrace result))
             (goto-char (point-min))
             (end-of-line)
             (let ((first-line (buffer-substring-no-properties
                                (point-min) (point))))
               (should (equal first-line
                              "  signal(ert-test-failed (\"foo\"))")))))))

This test creates a test object using make-ert-test whose body will immediately signal failure. It then runs that test and asserts that it fails. Then, it creates a temporary buffer and invokes ert--print-backtrace to print the backtrace of the failed test to the current buffer. Finally, it extracts the first line from the buffer and asserts that it matches what we expect. It uses buffer-substring-no-properties and equal to ignore text properties; for a test that takes properties into account, buffer-substring and ert-equal-including-properties could be used instead.

The reason why this test only checks the first line of the backtrace is that the remainder of the backtrace is dependent on ERT's internals as well as whether the code is running interpreted or compiled. By looking only at the first line, the test checks a useful property—that the backtrace correctly captures the call to signal that results from the call to ert-fail—without being brittle.

This example also shows that writing tests is much easier if the code under test was structured with testing in mind.

For example, if ert-run-test accepted only symbols that name tests rather than test objects, the test would need a name for the failing test, which would have to be a temporary symbol generated with make-symbol, to avoid side effects on Emacs's state. Choosing the right interface for ert-run-tests allows the test to be simpler.

Similarly, if ert--print-backtrace printed the backtrace to a buffer with a fixed name rather than the current buffer, it would be much harder for the test to undo the side effect. Of course, some code somewhere needs to pick the buffer name. But that logic is independent of the logic that prints backtraces, and keeping them in separate functions allows us to test them independently.

A lot of code that you will encounter in Emacs was not written with testing in mind. Sometimes, the easiest way to write tests for such code is to restructure the code slightly to provide better interfaces for testing. Usually, this makes the interfaces easier to use as well.

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4 How to Debug Tests

This section describes how to use ERT's features to understand why a test failed.

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4.1 Understanding Explanations

Failed should forms are reported like this:

     F addition-test
         (ert-test-failed
          ((should
            (=
             (+ 1 2)
             4))
           :form
           (= 3 4)
           :value nil))

ERT shows what the should expression looked like and what values its subexpressions had: The source code of the assertion was (should (= (+ 1 2) 4)), which applied the function = to the arguments 3 and 4, resulting in the value nil. In this case, the test is wrong; it should expect 3 rather than 4.

If a predicate like equal is used with should, ERT provides a so-called explanation:

     F list-test
         (ert-test-failed
          ((should
            (equal
             (list 'a 'b 'c)
             '(a b d)))
           :form
           (equal
            (a b c)
            (a b d))
           :value nil :explanation
           (list-elt 2
                     (different-atoms c d))))

In this case, the function equal was applied to the arguments (a b c) and (a b d). ERT's explanation shows that the item at index 2 differs between the two lists; in one list, it is the atom c, in the other, it is the atom d.

In simple examples like the above, the explanation is unnecessary. But in cases where the difference is not immediately apparent, it can save time:

     F test1
         (ert-test-failed
          ((should
            (equal x y))
           :form
           (equal a a)
           :value nil :explanation
           (different-symbols-with-the-same-name a a)))

ERT only provides explanations for predicates that have an explanation function registered. See Defining Explanation Functions.

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4.2 Interactive Debugging

Debugging failed tests essentially works the same way as debugging any other problems with Lisp code. Here are a few tricks specific to tests:

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5 Extending ERT

There are several ways to add functionality to ERT.

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5.1 Defining Explanation Functions

The explanation function for a predicate is a function that takes the same arguments as the predicate and returns an explanation. The explanation should explain why the predicate, when invoked with the arguments given to the explanation function, returns the value that it returns. The explanation can be any object but should have a comprehensible printed representation. If the return value of the predicate needs no explanation for a given list of arguments, the explanation function should return nil.

To associate an explanation function with a predicate, add the property ert-explainer to the symbol that names the predicate. The value of the property should be the symbol that names the explanation function.

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5.2 Low-Level Functions for Working with Tests

Both ert-run-tests-interactively and ert-run-tests-batch are implemented on top of the lower-level test handling code in the sections of ert.el labeled “Facilities for running a single test”, “Test selectors”, and “Facilities for running a whole set of tests”.

If you want to write code that works with ERT tests, you should take a look at this lower-level code. Symbols that start with ert-- are internal to ERT, whereas those that start with ert- are meant to be usable by other code. But there is no mature API yet.

Contributions to ERT are welcome.

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6 Other Testing Concepts

For information on mocks, stubs, fixtures, or test suites, see below.

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6.1 Other Tools for Emacs Lisp

Stubbing out functions or using so-called mocks can make it easier to write tests. See http://en.wikipedia.org/wiki/Mock_object for an explanation of the corresponding concepts in object-oriented languages.

ERT does not have built-in support for mocks or stubs. The package el-mock (see http://www.emacswiki.org/emacs/el-mock.el) offers mocks for Emacs Lisp and can be used in conjunction with ERT.

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6.2 Fixtures and Test Suites

In many ways, ERT is similar to frameworks for other languages like SUnit or JUnit. However, two features commonly found in such frameworks are notably absent from ERT: fixtures and test suites.

Fixtures are mainly used (e.g., in SUnit or JUnit) to provide an environment for a set of tests, and consist of set-up and tear-down functions.

While fixtures are a useful syntactic simplification in other languages, this does not apply to Lisp, where higher-order functions and `unwind-protect' are available. One way to implement and use a fixture in ERT is

     (defun my-fixture (body)
       (unwind-protect
           (progn [set up]
                  (funcall body))
         [tear down]))
     
     (ert-deftest my-test ()
       (my-fixture
        (lambda ()
          [test code])))

(Another way would be a with-my-fixture macro.) This solves the set-up and tear-down part, and additionally allows any test to use any combination of fixtures, so it is more flexible than what other tools typically allow.

If the test needs access to the environment the fixture sets up, the fixture can be modified to pass arguments to the body.

These are well-known Lisp techniques. Special syntax for them could be added but would provide only a minor simplification.

(If you are interested in such syntax, note that splitting set-up and tear-down into separate functions, like *Unit tools usually do, makes it impossible to establish dynamic `let' bindings as part of the fixture. So, blindly imitating the way fixtures are implemented in other languages would be counter-productive in Lisp.)

The purpose of test suites is to group related tests together.

The most common use of this is to run just the tests for one particular module. Since symbol prefixes are the usual way of separating module namespaces in Emacs Lisp, test selectors already solve this by allowing regexp matching on test names; e.g., the selector "^ert-" selects ERT's self-tests.

Other uses include grouping tests by their expected execution time, e.g., to run quick tests during interactive development and slow tests less often. This can be achieved with the :tag argument to ert-deftest and tag test selectors.

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Appendix A GNU Free Documentation License

Version 1.3, 3 November 2008
     Copyright © 2000, 2001, 2002, 2007, 2008, 2009 Free Software Foundation, Inc.
     http://fsf.org/
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.
  1. PREAMBLE

    The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

  2. APPLICABILITY AND DEFINITIONS

    This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

    A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

    A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

    The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.

    The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.

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    The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

    The “publisher” means any person or entity that distributes copies of the Document to the public.

    A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.

    The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.

  3. VERBATIM COPYING

    You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

    You may also lend copies, under the same conditions stated above, and you may publicly display copies.

  4. COPYING IN QUANTITY

    If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

    If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

    If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

  5. MODIFICATIONS

    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has fewer than five), unless they release you from this requirement.
    3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    4. Preserve all the copyright notices of the Document.
    5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
    8. Include an unaltered copy of this License.
    9. Preserve the section Entitled “History”, Preserve its Title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section Entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    11. For any section Entitled “Acknowledgements” or “Dedications”, Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.
    12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    13. Delete any section Entitled “Endorsements”. Such a section may not be included in the Modified Version.
    14. Do not retitle any existing section to be Entitled “Endorsements” or to conflict in title with any Invariant Section.
    15. Preserve any Warranty Disclaimers.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

    You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

    The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

  6. COMBINING DOCUMENTS

    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”

  7. COLLECTIONS OF DOCUMENTS

    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

  8. AGGREGATION WITH INDEPENDENT WORKS

    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.

  9. TRANSLATION

    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.

    If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.

  10. TERMINATION

    You may not copy, modify, sublicense, or distribute the Document except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, or distribute it is void, and will automatically terminate your rights under this License.

    However, if you cease all violation of this License, then your license from a particular copyright holder is reinstated (a) provisionally, unless and until the copyright holder explicitly and finally terminates your license, and (b) permanently, if the copyright holder fails to notify you of the violation by some reasonable means prior to 60 days after the cessation.

    Moreover, your license from a particular copyright holder is reinstated permanently if the copyright holder notifies you of the violation by some reasonable means, this is the first time you have received notice of violation of this License (for any work) from that copyright holder, and you cure the violation prior to 30 days after your receipt of the notice.

    Termination of your rights under this section does not terminate the licenses of parties who have received copies or rights from you under this License. If your rights have been terminated and not permanently reinstated, receipt of a copy of some or all of the same material does not give you any rights to use it.

  11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation. If the Document specifies that a proxy can decide which future versions of this License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Document.

  12. RELICENSING

    “Massive Multiauthor Collaboration Site” (or “MMC Site”) means any World Wide Web server that publishes copyrightable works and also provides prominent facilities for anybody to edit those works. A public wiki that anybody can edit is an example of such a server. A “Massive Multiauthor Collaboration” (or “MMC”) contained in the site means any set of copyrightable works thus published on the MMC site.

    “CC-BY-SA” means the Creative Commons Attribution-Share Alike 3.0 license published by Creative Commons Corporation, a not-for-profit corporation with a principal place of business in San Francisco, California, as well as future copyleft versions of that license published by that same organization.

    “Incorporate” means to publish or republish a Document, in whole or in part, as part of another Document.

    An MMC is “eligible for relicensing” if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

    The operator of an MMC Site may republish an MMC contained in the site under CC-BY-SA on the same site at any time before August 1, 2009, provided the MMC is eligible for relicensing.

ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

       Copyright (C)  year  your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.3
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:

         with the Invariant Sections being list their titles, with
         the Front-Cover Texts being list, and with the Back-Cover Texts
         being list.

If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.