title: compare two lists in python and return indices of matched values
body: <p>For two lists a and b, how can I get the indices of values that appear in both? For example,</p>
  
  <pre><code>a = [1, 2, 3, 4, 5]
  b = [9, 7, 6, 5, 1, 0]
  
  return_indices_of_a(a, b)
  </code></pre>
  
  <p>would return <code>[0,4]</code>, with <code>(a[0],a[4]) = (1,5)</code>.</p>