[numbers] Error checking

How much error checking do you expect? Are you going to throw us files that are not readable/non-existent? Or say a set of garbage numbers outside of the bounds of the contest? I.e., lines like this:
1,2,3,4,5,6,7,22222222
1,2,399,5,6,7
1,3,4,zork,6,7
etc.

Or should we just concentrate on the solution and not making the program robust. I don't want to code up a bunch of error handling and validation routines checking if speed is a determining factor and you won't throw us junk.

Well, judging by past experience, all inputs will be valid, but it's not necessary that a solution exists. So, all three of your examples will not happen, but something like this might:

1,2,3,4,5,6,999

xs0 is correct.