title: symbolically replace expressions in maxima
body: <p>I'm having trouble finding out how to do this:</p>
  
  <pre><code>x=a+b
  y=c+d
  z=x*y
  </code></pre>
  
  <p>I would like the output to be</p>
  
  <pre><code>z=ac+ad+bc+bd
  </code></pre>
  
  <p>not</p>
  
  <pre><code>z=xy
  </code></pre>