func (s *sim) disconnect(e graph.Edge) {
l := s.links[e]
l.sender.Close()
l.receiver.Close()
l.closed = true
s.links[e.Reverse()].closed = true
delete(s.links, e)
delete(s.links, e.Reverse())
}
func (s *sim) link(e graph.Edge) {
a := s.cs[e.A]
b := s.cs[e.B]
aconn := a.Link(b.id)
bconn := b.Link(a.id)
alink := &link{aconn, bconn, false}
blink := &link{bconn, aconn, false}
s.links[e] = alink
s.links[e.Reverse()] = blink
aconn.SetPendingFunc(func() { s.pending = append(s.pending, alink) })
bconn.SetPendingFunc(func() { s.pending = append(s.pending, blink) })
}