propagate 0s and 1s first 
	0	a : 1
	10	b : 0	[by a:=1]
	20	c : 1	[by b:=0]
testing overtaking of unstable rules 
	20	a : 0
queue:	30	b : 1	[from a:=0]
WARNING: unstable `b'+
rule: [after 10]	~a:1 -> b+:0
>> cause: `a' (val: 1)
	20	a : 1
No event pending on `b'.
