code: public boolean contains(Edge e) {
          EdgeSet<T> e1 = getEdgeSet(e.from());        
          return e1 != null && e1.contains(e);
      }
docs: {@inheritDoc}
  
  <p> This method is sensitive to the vertex ordering; a call will check
  whether the edge set for {@code e.from()} contains {@code e}.  Subclasses
  should override this method if their {@link EdgeSet} implementations are
  sensitive to the ordering of the vertex indices, or if a more advanced
  behavior is needed.