enqueued:	10	c : 1	[from _p:=0]
	10	c : 1	[by _p:=0]
time: 10
enqueued:	20	c : X	[from a:=X]
time: 10
queue:	20	c : X	[from a:=X]
queue:	20	c : 0	[from b:=X]
WARNING: weak-unstable `c'-
rule: [after 10]	a:X -> c-:1
>> cause: `b' (val: 0)
weak-unstable should leave x-pull in queue 
queue:	20	c : X	[from b:=X]
	20	c : X	[by b:=0]
