Answer set with two abductive supports is duplicated

[vuphan314]

@iensen :grin:

Below is a CR-Prolog program:

predicates
a(). b(). c().

rules

a.

-a :- not b, not c.
b :+. c :+.

b :- c.
c :- b.

Sparc+DLV gives:

SPARC  V2.49
program translated
{a, b, c}

{a, b, c}

The answer set {a, b, c} is duplicated, possibly because it has two abductive supports {b :+.} and {c :+.}.

[iensen]

When using Clingo solver, this can be fixed by using #project directive (see my fix for https://github.com/iensen/plog2.0/issues/18).

I am not quite sure about DLV, perhaps we can decide not to support it anymore.