Consider the loop conditions (which are clearly non-trivial) and ask "is it certain that this loop will always terminate"? Presumably in this case you would ha

Consider the loop conditions (which are clearly non-trivial) and ask "is it certain that this loop will always terminate"? Presumably in this case you would have arrived at an answer along the lines of "yes, so long as there's always a sunset". Which should then prompt the question: hang on, is there always a sunset? A quick scan over whatever datafile it's using for input should answer that.