time: 0
	0	a : 1
expect instability 
WARNING: unstable `b'-
rule: [after 10]	a:0 -> b-:X
>> cause: `a' (val: 0)
	5	a : 0
event queue:
	10	b : X	[from a:=1]
expect b:X in queue 
end of queue 
	0	a : 0
expect instability 
WARNING: unstable `b'+
rule: [after 10]	~a:1 -> b+:X
>> cause: `a' (val: 1)
	5	a : 1
event queue:
	10	b : X	[from a:=0]
