code: protected function buildOn()/*# : array */
      {
          $result = [];
          foreach ($this->clause_on as $on) {
              $res = [ $on[0] ? 'OR' : 'ON'];
              $res[] = $this->quote($on[1]); // first col
              $res[] = $on[2]; // operator
              $res[] = $this->quote($on[3]); // second col
              $result[] = join(' ', $res);
          }
          return $result;
      }
docs: Build ON
  
  @return array
  @access protected