NullableRange

assert(NullableRange!int(99).front == 99);

auto x = NullableRange!int(100);
assert(x.front == 100);

x.front = 1234;
assert(x.front == 1234);
x.popFront;
assert(x.empty == true);

auto x = NullableRange!int(99).asNullable;
assert(!x.isNull);
assert(x.get == 99);
x.nullify();
assert(x.isNull);