fixup! Use sets

            sequence = []

        # Skip already validated branches

        if validated.get(element) is not None:

        if element in validated:

            return

        if check_elements.get(element) is not None:

        if element in check_elements:

            # Create `chain`, the loop of element dependencies from this

            # element back to itself, by trimming everything before this

            # element from the sequence under consideration.