// Finds an edge source slot in the output graph. If the edge crosses a
  // subgraph boundary it is a slot on the output of a call node, otherwise it
  // is a slot on a node in the output graph.
  int FindOutputSlotOfEdgeSrc(const string& src_func_id,
                              const string& dst_func_id,
                              const Edge* edge);

  // Finds the image of an edge destination in the output graph. If the edge