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The beginning of a comment is indicated with a semicolon (;). Scheme ignores everything on a line in which a semicolon appears, from the semicolon until the end of the line. The entire comment, including the newline character that terminates it, is treated as whitespace.

An alternative form of comment (sometimes called an extended comment) begins with the characters ‘#|’ and ends with the characters ‘|#’. This alternative form is an MIT/GNU Scheme extension. As with ordinary comments, all of the characters in an extended comment, including the leading ‘#|’ and trailing ‘|#’, are treated as whitespace. Comments of this form may extend over multiple lines, and additionally may be nested (unlike the comments of the programming language C, which have a similar syntax).

;;; This is a comment about the FACT procedure.  Scheme
;;; ignores all of this comment.  The FACT procedure computes
;;; the factorial of a non-negative integer.

This is an extended comment.
Such comments are useful for commenting out code fragments.

(define fact
  (lambda (n)
    (if (= n 0)                      ;This is another comment:
        1                            ;Base case: return 1
        (* n (fact (- n 1))))))