code: def _filter_subgraph(self, subgraph, predicate):
          
          to_return = []
  
          for unique_id, item in subgraph.items():
              if predicate(item):
                  to_return.append(item)
  
          return to_return
docs: Given a subgraph of the manifest, and a predicate, filter
          the subgraph using that predicate. Generates a list of nodes.