title: Replace first occurrence of symbol in (possibly nested) list
body: <p>I would like to replace just the first occurrence of a certain symbol (say <code>'-</code>) with another symbol (say <code>'+</code>) inside a list that may contain lists. That is to say, </p>
  
  <p><code>'(((-)))</code> would turn into <code>'(((+)))</code></p>
  
  <p><code>'((-) - b)</code> into <code>'((+) - b)</code></p>