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