	0	a : 0
	10	c : 0	[by a:=0]
WARNING: weak-interference `c'
>> cause: `a' (val: 1)
	10	a : 1
	20	c : X	[by a:=1]
	20	b : 1
	30	c : 1	[by b:=1]
	30	b : 0
	40	c : 0	[by b:=0]
	40	a : X
WARNING: weak-interference `c'
>> cause: `b' (val: X)
	40	b : X
	50	c : X	[by b:=X]
time: 50
