s(CASP) is said to work over an HU (Herbrand universe) which is an extension of HU_0 (the HU of the program).
So program
p(0).
p(s(_)).
p(X) :- not p(X).
does not have a stable model (it has one over HU_0).
However s(CASP) run under Swish does not notice this, finds the same two answers as Prolog does, and displays s(CASP) model as
p(0)
p(s(A))
p(s(B))
Bug or feature?