Given a subgraph of the manifest, and a predicate, filter
        the subgraph using that predicate. Generates a list of nodes.

def _filter_subgraph(self, subgraph, predicate):
        """
        Given a subgraph of the manifest, and a predicate, filter
        the subgraph using that predicate. Generates a list of nodes.
        """
        to_return = []

        for unique_id, item in subgraph.items():
            if predicate(item):
                to_return.append(item)

        return to_return