code: def get_peripheral_successor_edges(graph: BELGraph, subgraph: BELGraph) -> EdgeIterator:
      
      for u in subgraph:
          for _, v, k in graph.out_edges(u, keys=True):
              if v not in subgraph:
                  yield u, v, k
docs: Get the set of possible successor edges peripheral to the sub-graph.
  
      The source nodes in this iterable are all inside the sub-graph, while the targets are outside.