Adds a match for the elements avaialble at lst_idx and root_idx.

def add_root_match(self, lst_idx, root_idx):
        """Adds a match for the elements avaialble at lst_idx and root_idx."""
        self.root_matches[lst_idx] = root_idx
        if lst_idx in self.in_result_idx:
            return

        self.not_in_result_root_match_idx.add(lst_idx)