Merges the given Item with the last one currently in the given Container if
        both are whitespace items.

        Returns True if the items were merged.

def _merge_ws(self, item, container):  # type: (Item, Container) -> bool
        """
        Merges the given Item with the last one currently in the given Container if
        both are whitespace items.

        Returns True if the items were merged.
        """
        last = container.last_item()
        if not last:
            return False

        if not isinstance(item, Whitespace) or not isinstance(last, Whitespace):
            return False

        start = self._idx - (len(last.s) + len(item.s))
        container.body[-1] = (
            container.body[-1][0],
            Whitespace(self._src[start : self._idx]),
        )

        return True