xnor method
Combines this spec with another using XNOR logic (NOT XOR / equivalence).
Returns a new spec that is satisfied when both specs have the same satisfaction state: either both satisfied or both unsatisfied. Equivalent to calling xor followed by negated.
Prefer someSpec.xor(other).negated() when readability matters.
Use xnor as a convenience when the equivalence intent is central
to the business rule.
Example:
// True when both conditions agree (both true or both false)
final syncedState = IsVerified().xnor(IsApproved());
Implementation
@nonVirtual
Spec<T> xnor(Spec<T> other) => _Xnor(this, other);