	0	a : 0
	10	b : 1	[by a:=0]
	20	c : 0	[by b:=1]
WARNING: weak-interference `b'
>> cause: `a' (val: X)
	20	a : X
Warning: possible invariant violation in [top-level]: (b:X) by node b:X
WARNING: weak-interference `c'
>> cause: `b' (val: X)
	30	b : X	[by a:=X]
	40	c : X	[by b:=X]
