code: public function constraint(/*# string */ $string)
      {
          if (!isset($this->tbl_constraint['other'])) {
              $this->tbl_constraint['other'] = [];
          }
          $this->tbl_constraint['other'][] = $string;
          return $this;
      }
docs: Create a constraint
  
  @param  string $string
  @return self
  @access public