code: public function colConstraint(/*# string */ $string)
      {
          $col = &$this->col_defs[count($this->col_defs) - 1];
          if (!isset($col['constraint'])) {
              $col['constraint'] = [];
          }
          $con = &$col['constraint'];
          $con[] = $string;
          return $this;
      }
docs: {@inheritDoc}