title: "set!: not an identifier in:..." plt scheme error
body: <p>what's wrong with this code in Dr.Scheme using Pretty Big? i seem to remember doing similar things in the past with no problem.</p>
  
  <pre><code>(lambda (x y) (set! (Frame-variables res) (append (Frame-variables res) (list (cons x y)))))
  </code></pre>
  
  <p>which returns the following error:</p>
  
  <blockquote>
    <p><code>set!: not an identifier in: (Frame-variables res)</code></p>
  </blockquote>
  
  <p>if i omit the <code>(set! (Frame-variables res) ... )</code> part it works fine except of course that my list doesn't actually change. so it recognizes the second <code>(Frame-varialbes res)</code> but not the first one? or what's the deal with <code>set!</code> in that context? </p>
  
  <p>hopefully you recognize that i have a struct called <code>Frame</code> with a <code>variables</code> field which is a list.</p>