Set differences

[Josh]

Can anyone please explain why this works:

        auto numbers = sequence!("n")();
	auto trimmed = setDifference(setDifference(numbers, sequence!("n 
* a[0]")(2)), sequence!("n * a[0]")(3));

but this doesn't?

        auto numbers = sequence!("n")();
        auto trimmed = setDifference(numbers, sequence!("n * a[0]")(2));
        trimmed = setDifference(trimmed, sequence!("n * a[0]")(3));

Thanks,
Josh