	0	a : 0
	10	b : 1	[by a:=0]
	10	a : 1
	20	b : 0	[by a:=1]
Fanouts of node `a':
(to [top-level]):
[after 10, weak]	a:1 -> b-:0
[after 10, weak]	~a:1 -> b+:0
Fanins of node `b':
(from [top-level]):
[after 10, weak]	~a:1 -> b+:0
[after 10, weak]	a:1 -> b-:0
	20	a : 0
	30	b : 1	[by a:=0]
	30	a : 1
	40	b : 0	[by a:=1]
expect no effect of changing a 
	40	a : 0
	40	a : 1
Fanouts of node `a':
(to [top-level]):
Fanins of node `b':
(from [top-level]):
	40	a : 0
	40	a : 1
