Fanins of node `b':
(from [top-level]):
[after 10]	~a -> b+
[after 10]	a -> b-
Fanins of node `a':
(from [top-level]):
[after 0, weak]	~b -> a+
[after 0, weak]	b -> a-
event queue:
	10	b : 1	[from a:=0]
event queue:
	20	b : 0	[from a:=1]
event queue:
time: 20
