title: Quadratic formula in Scheme
body: <p>Scheme</p>
  
  <p>I want to define a function which 
  returns the roots of the quadratic formula, given
   ax^2+bx+c=0. Return only real roots. The list will
   have 0, 1, or two unique roots.</p>
  
  <pre><code>(define (quadratic a b c)
      (cond
          ((&gt; (- (* b b) (* 4 (* a c ) ) ) 0 ) (list ( / ( - (sqrt ( - (* b b) (* (* 4 a) c))) b ) ( * 2 a) )
              ( / ( - ( - (sqrt ( - (* b b) (* (* 4 a) c)))) b ) ( * 2 a) ) ) )
          ((= (- (* b b) (* 4 (* a c ) ) ) 0 ) list( / ( - (sqrt ( - (* b b) (* (* 4 a) c))) b ) ( * 2 a) ))
          (else ('( )'))
      )
  )
  </code></pre>
  
  <p>I got</p>
  
  <pre><code>error: unexpected right parenthesis [read]
         #{&amp;read-error}
         #{&amp;i/o-port-error #{input-port #{input-channel "standard input" 0}}}
  </code></pre>
  
  <p>Is there any better way to solve that?</p>