time: 0
setting 'a-' and letting propagate 
	0	a : 0
	10	b : 1	[by a:=0]
	20	c : 0	[by b:=1]
introduce a low pulse on 'a' 
	20	a : 1
expect instability on b- because of a- 
WARNING: unstable `b'-
rule: [after 10]	a:0 -> b-:1
>> cause: `a' (val: 0)
	25	a : 0
WARNING: weak-interference `c'
>> cause: `b' (val: X)
	30	b : X	[by a:=0]
	40	c : X	[by b:=X]
bring back to known states 
	40	a : 1
	50	b : 0	[by a:=1]
	60	c : 1	[by b:=0]
introduce a high pulse on 'a' 
	60	a : 0
expect instability on b+ because of a+ 
WARNING: unstable `b'+
rule: [after 10]	~a:1 -> b+:0
>> cause: `a' (val: 1)
	65	a : 1
WARNING: weak-interference `c'
>> cause: `b' (val: X)
	70	b : X	[by a:=1]
	80	c : X	[by b:=X]
