title: How to write a prolog program for recursive subtraction
body: <p>I want a recursive subtraction program in prolog.</p>
  <p>the scheme of the program is</p>
  <pre><code>(define (sub m n)
  (cond
      ((= n 0) m)
      (else (sub (- m 1) (- n 1)))))
  </code></pre>
  <p>I need to convert this program into a prolog program..</p>