broken puzzle

[DaKnig]

I am solving "Merge-step" currently.
unless I am overlooking the obvious, there's no way to know when one sequence is over.
my code gets stuck when one of the sequences is depleted; in a normal sorting algorithm, you always have a loop over the remaining items of the other buffer in such cases, but there's no way for the code to know if the source of the sequences is done, without a hack. in most levels, it's "sequence is zero terminated".