def __init__(self, sort_key=None):
        """
        Create a new empty BST.  If a sort key is specified, then it
        will be used to define the sort order for the BST.  If an
        explicit sort key is not specified, then each value is
        considered its own sort key.
        """
        self._root = [] # = empty node
        self._sort_key = sort_key
        self._len = 0 # keep track of how many items we contain.

        Create a new empty BST.  If a sort key is specified, then it
        will be used to define the sort order for the BST.  If an
        explicit sort key is not specified, then each value is
        considered its own sort key.
        
recipes/Python/577830_Python_Binary_Search_Tree/recipe-577830.py