def __len__(self):
def _len_help(tree):<EOL><INDENT>n = sum(_len_help(t[<NUM_LIT:1>]) for t in tree['<STR_LIT>'])<EOL>return n + <NUM_LIT:1> if tree['<STR_LIT>'] is not None else n<EOL><DEDENT>return _len_help(self.root)<EOL>
Effective number of keys in the tree.  Note that inserting keys
that override other keys leads to the overridden keys being
purged from the tree, so the key count does not necessarily
increase monotonically as keys are inserted.