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The ligature information for a character in an encoding file is optional. More than one ligature specification may be given. Each specification looks like:
lig second-char =: lig-char
This means that a ligature character lig-char should be present in the font for the current character (the one being defined on this line of the encoding file) followed by second-char. You give second-char and lig-char as character codes (see section 3.3.3 Specifying character codes). For example, in most text encodings (which involve Latin characters), some variation on the following line will be present:
f lig f =: 013 lig i =: 014 lig l =: 015
This will produce a ligature in the font such that when a typesetting program sees the two character sequence `ff' in the input, it replaces those two characters in the output with the single character at position octal 13 (presumably the `fi' ligature) of the font; when it sees `fi', the character at position octal 14 is output; when it sees `fl', the character at position octal 15 is output.
Metafont version 2 allows a more general ligature scheme; if there is a demand for it, it wouldn't be hard to add.