title: Occurence of characters in common in two strings
body: <p>I want to use a <em>for</em> loop to calculate the number of times a character in one string occurs in another string.</p>
  
  <p>e.g. if string1 = 'python' and string2 = 'boa constrictor' then it should calculate to 6 (2 t's, 3 o's, 1 n)</p>
  
  <p>Does anyone know how to do this?</p>