title: "unbound identifier" errors in scheme
body: <p>I'm using drscheme from:
  <a href="http://www.archlinux.org/packages/extra/x86_64/drscheme/" rel="noreferrer">http://www.archlinux.org/packages/extra/x86_64/drscheme/</a></p>
  
  <p>I'm trying to work with the sample code in my textbook, but I keep getting getting "unbound identifier" errors. Is it because the scheme interpreter is not configured correctly? or is the code just plain wrong?</p>
  
  <p>Here are a few examples:</p>
  
  <p><strong>Input:</strong></p>
  
  <pre><code>#lang scheme
  (define (equalimp lis1 lis2)
          (COND
           ((NULL? lis1) (NULL? lis2))
           ((NULL? lis2) '())
           ((EQ? (CAR lis1) (CAR lis2)) (equalimp (CDR lis1) (CDR lis2)))
           (ELSE '())
  ))
  </code></pre>
  
  <p><strong>Output:</strong></p>
  
  <p>Welcome to DrScheme, version 4.2.5 [3m]. Language: scheme; memory limit: 128 MB.</p>
  
  <p><strong>expand: unbound identifier in module in: COND</strong></p>
  
  <p><strong>Input:</strong></p>
  
  <pre><code>#lang scheme
  (define (quadratic_roots a b c)
    (LET (
          (root_part_over_2a
           (/ (SQRT (- (* b b) (* 4 a c))) (* 2 a)))
          (minus_b_over_2a (/ (- 0 b) (* 2 a)))
         )
    (DISPLAY (+ minus_b_over_2a root_part_over_2a))
    (NEWLINE)
    (DISPLAY (- minus_b_over_2a root_part_over_2a))
    ))
  </code></pre>
  
  <p><strong>Output:</strong></p>
  
  <p><strong>expand: unbound identifier in module in: LET</strong></p>
  
  <p>Note: I tried using LET* because I read this: stackoverflow.com/ questions/946050/using-let-in-scheme but it produces the same error. </p>
  
  <p>Thanks !</p>