def _remove_aux(self, node, key):
        """
        -------------------------------------------------------
        Attempts to find a value matching key in a BST node. Deletes the node
        if found and returns the sub-tree root.
        Private recursive operation called only by remove.
        Use: node, value = self._remove_aux(node, key)
        -------------------------------------------------------
        Preconditions:
            node - a bst node to search for key (_BSTNode)
            key - data to search for (?)
        Postconditions:
            returns
            node - the current node or its replacement (_BSTNode)
            value - value in node containing key, None otherwise.
        -------------------------------------------------------
        """

        if node is None:
            value = None
        elif key < node._data:
            node._left, value = self._remove_aux(node._left, key)
        elif key > node._data:
            node._right, value = self._remove_aux(node._right, key)
        else:
            value = node._data
            if node._left is None:
                node = node._right
            elif node._right is None:
                node = node._left
            else:
                if node._left._right is None:
                    repl_node = node._left
                else:
                    repl_node = self._delete_node_left(
                        node._left, node._left._right)
                    repl_node._left = node._left
                repl_node._right = node._right
                node = repl_node
                node._update_height()
            self._count -= 1
        if node is not None and value is not None:
            node._update_height()

        return node, value