code: public function toSql()
      {
          if (empty($this->into) === true) {
              throw new BadUse('into clause has not been defined (' . print_r($this, true) . ')');
          }
          $sql = 'INSERT INTO ' . $this->into . ' ';
  
          if ($this->values[0] != null) {
              $sql .= '(' . $this->values[0] . ') ';
          }
          if ($this->values[1] != null) {
              $sql .= 'VALUES (' . $this->values[1] . ') ';
          } else {
              $sql .= 'DEFAULT VALUES ';
          }
  
          if ($this->returning != null) {
              $sql .= 'RETURNING ' . $this->returning;
          }
  
          return $sql;
      }
docs: Extract the corresponding SQL code.
  
  @throws BadUse if into clause has not been defined
  @return string generated SQL code