code: function addCharacters(regen, set) {
      // Add the character range, or a single character itself, to the set
      if (set[1] !== undefined) {
        regen.addRange(set[0], set[1]);
      } else {
        regen.add(set[0]);
      }
  
      return regen;
    }
docs: Adds the given character/range to the set
  
  @param {Regenerate} regen An instance of the Regenerate object
  @param {Array} set An Array containing a 0th and (optionally) a 1st index
  representing a character or character range
  @returns {Regenerate} An updated instance of the Regenerate object