title: Iterate a list using recursion
body: <p>How can I write this same function using recursion?</p>
  
  <pre><code>def replace(thelist,a,b):
  """Returns: a COPY of thelist but with all occurrences of a replaced by b. 
  
      Example: replace([1,2,3,1], 1, 4) = [4,2,3,4].
  
  Precondition: thelist is a list of ints
                a and b are ints"""
  </code></pre>
  
  <p>I believe I'm supposed to make a copy of the original, and then from there make a = b in the list, but I do not know how to implement that. Please help me solve this, and thank you!</p>