title: Change last element in list
body: <p>I have a nested list: (def mylist '(+ '(+ 1 2) 3))</p>
  
  <p>When I eval it, I get 6 as expected: (eval mylist)</p>
  
  <p>How can I change the last element (3)? I'd like to get a list like this:</p>
  
  <pre><code>(+ (+ 1 2) 4)
  </code></pre>
  
  <p>I've tried</p>
  
  <pre><code>(list (butlast mylist) 4)
  </code></pre>
  
  <p>but it yields this:</p>
  
  <pre><code>((+ (+ 1 2)) 4)
  </code></pre>
  
  <p>i.e. the first part is nested in a list itself. How can I fix it? I'm building up that list datastructure recursively and at a later point I'll evaluate it via the eval function.</p>