code: public function range() {
          $arg_num = func_num_args();
          if ($arg_num % 2 != 0) {
              throw new \InvalidArgumentException("Number of args must be even", 1);
          }
          $value = "[";
          $arg_list = func_get_args();
          for ($i = 0; $i < $arg_num;) {
              $value .= self::sanitize($arg_list[$i++]) . "-" . self::sanitize($arg_list[$i++]);
          }
          $value .= "]";
          return $this->add($value);
      }
docs: Add a range
  
  Adds a range to our expression ex: range(a,z) => a-z, range(a,z,0,9) => a-z0-9
  
  @access public
  @return VerbalExpressions
  @throws \InvalidArgumentException