title: Count number of unique element in a list
body: <p>Say I have a list, <code>a b c b b d e e f …</code>, and I don't know how many different kind of elements are in there. </p>
  
  <p>How do I count the number of each unique element and print them out?
  Output would looks like:</p>
  
  <pre>
  a: 32
  b: 12
  c: 6
  …
  </pre>