code: function DoubleLast(input, letters) {
      var last = LastChar(input);
      if (!letters || ~letters.indexOf(last)) {
          return input + last;
      }
      return input;
  }
docs: DoubleLast
  Doubles the last letter.
  
  @name DoubleLast
  @function
  @param {String} input The input string.
  @param {Array} letters An array of letters: if the last letter of the input
  is not found in this list, it will *not* be doubled.
  @return {String} The modified string.