	0	a : 0
	10	b : 1	[by a:=0]
	20	c : 0	[by b:=1]
	20	a : 1
event queue:
	30	b : 0	[from a:=1]
time: 24
expect weak-instability on b- 
WARNING: weak-unstable `b'-
rule: [after 10]	a:X -> b-:1
>> cause: `a' (val: X)
WARNING: weak-interference `b'
>> cause: `a' (val: X)
	24	a : X
WARNING: weak-interference `c'
>> cause: `b' (val: X)
	30	b : X	[by a:=X]
	40	c : X	[by b:=X]
event queue:
