reverse method
Return a Semigroup
that reverses the order.
final instance = Semigroup.instance<String>((a1, a2) => '$a1$a2');
final reverse = instance.reverse();
expect(reverse.combine('a', 'b'), 'ba');
expect(reverse.combine('a', 'b'), instance.combine('b', 'a'));
Implementation
Semigroup<T> reverse() => _Semigroup((x, y) => combine(y, x));