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[racket] Pattern matching define macro

From: Daniel Prager (daniel.a.prager at gmail.com)
Date: Sat Jul 12 18:19:27 EDT 2014		 
Hi Brian

r >= 0 && r <= 4 && c >= 0 && c <= r


implies

0 <= c <= r <= 4


Or  using prefix and the variable-arity of <=:

(define (is-pos r c)
  (<= 0 c r 4))


which I think works well for clarity, concision, and efficiency.

Dan
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