public static bool HasIdentity <MONOID, T>(T x, IEqualityComparer <T> comparer = null)
where MONOID : struct, IMonoid <T>
{
comparer ??= EqualityComparer <T> .Default;
return(comparer.Equals(Monoid.Concat <MONOID, T>(x, Monoid.Empty <MONOID, T>()), x) &&
comparer.Equals(Monoid.Concat <MONOID, T>(Monoid.Empty <MONOID, T>(), x), x));
}