code: public function offsetGet($offset)
      {
          $offset = $this->calculateOffset($offset);
          return isset($this->_children[$offset]) ? $this->_children[$offset] : null;
      }
docs: Returns a child token at the specified offset.
  This method is required by the SPL [[\ArrayAccess]] interface.
  It is implicitly called when you use something like `$child = $token[$offset];`.
  @param int $offset child token offset.
  @return SqlToken|null the child token at the specified offset, `null` if there's no token.