formulas(sos).
all x all y all z (Sum(Sum(x,y),z) = Sum(x,Sum(y,z))).
all x all y (IsIn(x,y) <-> (exists z (Sum(x,z) = y))).
end_of_list.

formulas(goals).
all x all y all z (IsIn(y,z) -> IsIn(Sum(x,y),Sum(x,z))).
end_of_list.