text: /**
   * Write a function `f(a, b)` which takes two strings as arguments and returns a
   * string containing the characters found in both strings (without duplication), in the
   * order that they appeared in `a`. Remember to skip spaces and characters you
   * have already encountered!
   *
   * Example: commonCharacters('acexivou', 'aegihobu')
   * Returns: 'aeiou'
   *
   * Extra credit: Extend your function to handle more than two input strings.
   */
  
  
  
  var commonCharacters = function(string1, string2) {
    // TODO: Your code here!
  };
  