Optimization to only ground relevant rule instances interferes with the backend #248
Description
When grounding the program below, the instance :- a(2). of :- a(X). is wrongly discarded resulting in the superfluous answer set { a(2) }.
>>> from clingo import Function, Control, Number
>>>
>>> c = Control()
>>> c.configuration.solve.models = "0"
>>> with c.backend() as backend:
... backend.add_rule([backend.add_atom(Function("a", [Number(1)]))], choice=True)
... backend.add_rule([backend.add_atom(Function("a", [Number(2)]))], choice=True)
...
>>> c.add("base", [], ":- a(X).")
>>> c.ground([("base", [])])
>>> c.solve(on_model=print)
a(2)
SAT