code: public function add(string $sql)
      {
          $args = func_get_args();
          array_shift($args); // drop the first one (the sql string)
          $this->sql .= $sql;
          $this->args = array_merge($this->args, $args);
          return $this;
      }
docs: appends sql and args to query
  @param string $sql
  @return $this