Prune/merge all nodes that are smaller than small_size.

def prune_if_smaller_than(self, small_size, a_or_u):
        "Prune/merge all nodes that are smaller than small_size."
        if self._prune_all_if_small(small_size, a_or_u):
            return

        for node in self._nodes:
            node.prune_if_smaller_than(small_size, a_or_u)

        self._prune_some_if_small(small_size, a_or_u)