Lisp ignores the rest of a line after a ; character. This feature allows comments to be included in a program.

   ;;; edited 25 Aug 2000 by GSN
   ;;; Compute factorial of an integer n.
   ;;; n should be non-negative.
   ;;; 1 is returned for negative numbers.
   (define (factorial n)
     (if (<= n 0)          ; <= for safety
         1                 ; base case
         (* n (factorial (- n 1))) ) )

Document your code well with comments that describe:

• The types of inputs
• Assumptions made about inputs
• An example call with inputs
• The algorithm used
• Performance
• The results produced

Rule: Document unto others as you would have others document unto you.

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