Determine the transitive closure of tkn's dependencies
        :param tkn: root token
        :param seen: list of tokens already visited in closure process
        :return: dependents, dependents of dependents, etc.

def dependency_closure(self, tkn: str, seen: Optional[Set[str]]=None) -> Set[str]:
        """
        Determine the transitive closure of tkn's dependencies
        :param tkn: root token
        :param seen: list of tokens already visited in closure process
        :return: dependents, dependents of dependents, etc.
        """
        if seen is None:
            seen = set()
        for k in self.dependencies(tkn):
            if k not in seen:
                seen.add(k)
                self.dependency_closure(k, seen)
        return seen