eitherEitherIdentity

static class TestedFunction : Function!(int, string)
{
	override string opCall(int x)
	{
		import std.math, std.conv;
		return sqrt(real(1) + x * x).to!string;
	}
	mixin Singleton;
}
auto t = TestedFunction.get;
auto
	t0 = t.functionTupleIdentity,
	t1 = t.tupleTupleIdentity!(int[]),
	t2 = t.eitherFunctionIdentity,
	t3 = t.eitherEitherIdentity!(int[]);
static assert (is (typeof (t0) : Function!(int, Tuple!(string, int))));
static assert (is (typeof (t1) : Function!(Tuple!(int, int[]), Tuple!(string, int[]))));
static assert (is (typeof (t2) : Function!(Either!(int, string), string)));
static assert (is (typeof (t3) : Function!(Either!(int, int[]), Either!(string, int[]))));