CLHS: Section 2.4.8.19.1

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## 2.4.8.19.1 Examples of Sharpsign Vertical-Bar

The following are some examples that exploit the `#`|`...`|`#` notation:

    ;;; In this example, some debugging code is commented out with #|...|#
    ;;; Note that this kind of comment can occur in the middle of a line
    ;;; (because a delimiter marks where the end of the comment occurs)
    ;;; where a semicolon comment can only occur at the end of a line
    ;;; (because it comments out the rest of the line).
     (defun add3 (n) #|(format t "~&Adding 3 to ~D." n)|# (+ n 3))

    ;;; The examples that follow show issues related to #| ... |# nesting.

    ;;; In this first example, #| and |# always occur properly paired,
    ;;; so nesting works naturally.
     (defun mention-fun-fact-1a ()
       (format t "CL uses ; and #|...|# in comments."))
    =>  MENTION-FUN-FACT-1A
     (mention-fun-fact-1a)
    >>  CL uses ; and #|...|# in comments.
    =>  NIL
     #| (defun mention-fun-fact-1b ()
          (format t "CL uses ; and #|...|# in comments.")) |#
     (fboundp 'mention-fun-fact-1b) =>  NIL

    ;;; In this example, vertical-bar followed by sharpsign needed to appear
    ;;; in a string without any matching sharpsign followed by vertical-bar
    ;;; having preceded this.  To compensate, the programmer has included a
    ;;; slash separating the two characters.  In case 2a, the slash is
    ;;; unnecessary but harmless, but in case 2b, the slash is critical to
    ;;; allowing the outer #| ... |# pair match.  If the slash were not present,
    ;;; the outer comment would terminate prematurely.
     (defun mention-fun-fact-2a ()
       (format t "Don't use |\# unmatched or you'll get in trouble!"))
    =>  MENTION-FUN-FACT-2A
     (mention-fun-fact-2a)
    >>  Don't use |# unmatched or you'll get in trouble!
    =>  NIL
     #| (defun mention-fun-fact-2b ()
          (format t "Don't use |\# unmatched or you'll get in trouble!") |#
     (fboundp 'mention-fun-fact-2b) =>  NIL

    ;;; In this example, the programmer attacks the mismatch problem in a
    ;;; different way.  The sharpsign vertical bar in the comment is not needed
    ;;; for the correct parsing of the program normally (as in case 3a), but
    ;;; becomes important to avoid premature termination of a comment when such
    ;;; a program is commented out (as in case 3b).
     (defun mention-fun-fact-3a () ; #|
       (format t "Don't use |# unmatched or you'll get in trouble!"))
    =>  MENTION-FUN-FACT-3A
     (mention-fun-fact-3a)
    >>  Don't use |# unmatched or you'll get in trouble!
    =>  NIL
     #|
     (defun mention-fun-fact-3b () ; #|
       (format t "Don't use |# unmatched or you'll get in trouble!"))
     |#
     (fboundp 'mention-fun-fact-3b) =>  NIL

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