testing overtaking of unstable rules 
	0	a : 0
queue:	10	b : 1	[from a:=0]
WARNING: unstable `b'+
rule: [after 10]	~a:1 -> b+:X
>> cause: `a' (val: 1)
	0	a : 1
queue:	10	b : 0	[from a:=1]
	10	b : 0	[by a:=1]
	20	c : 1	[by b:=0]
