title: GCD of two numbers using Racket
body: <p>When I use racket to define a function that find the gcd of two integers,</p>
  
  <pre><code>(define (gcd a b)
      (cond
           [(&gt; a b) (gcd b a-b)]
           [(&lt; a b) (gcd a b-a)]
           [else a]))
  </code></pre>
  
  <p>But, </p>
  
  <blockquote>
    <p>a-b: unbound identifier in module in: a-b</p>
  </blockquote>
  
  <p>I don't know what to do.
  Why is that happen?</p>