code: def as_graph_queue(self, manifest, limit_to=None):
          
          if limit_to is None:
              graph_nodes = self.graph.nodes()
          else:
              graph_nodes = limit_to
  
          new_graph = _subset_graph(self.graph, graph_nodes)
          return GraphQueue(new_graph, manifest)
docs: Returns a queue over nodes in the graph that tracks progress of
          dependecies.