Purge unreachable dominated key paths before inserting a new key
        path.

def _purge_unreachable(self, key):
        """
        Purge unreachable dominated key paths before inserting a new key
        path.

        """
        dels = []
        for p in self:
            if dominates(key, p):
                dels.append(p)
        for k in dels:
            _, idxs = find_in_tree(self.root, k, perfect=True)
            del_by_idx(self.root, idxs)