auto a = Result!int.ok(3);
auto b = Result!int.err("failure");
assert(a.or(b) is a);
assert(b.or(a) is a);
a = Result!int.err("failure");
b = Result!int.err("late failure");
assert(a.or(b) is b);
a = Result!int.ok(3);
b = Result!int.ok(10);
assert(a.or(b) is a);