public static void IntMonoidTest()
{
var intMult = new Monoid <int>(1, (x, y) => x * y);
Assert.Equal(5, intMult.Op(intMult.Neutral, 5));
Assert.Equal(5, intMult.Op(5, intMult.Neutral));
Assert.Equal(15, intMult.Op(3, 5));
}