	0	a : 0
event queue:
	10	b : 1	[from a:=0]
WARNING: pending event for node `b' was overridden.
set b 0 accepted 
event queue:
	5	b : 0
continuing... 
	5	b : 0
	15	c : 1	[by b:=0]
kind of incoherent with a- and b-... 
what happens if we set a- now? should b+ be queued? 
time: 15
event queue:
nothing, vacuous event suppressed 
a : 0
b : 0
re-evaluate all nodes 
	25	b : 1
	35	c : 0	[by b:=1]
