Given (on STDIN, as command line arguments, or as function arguments) two distinct non-empty strings, find and return the shortest substring of the first string which is not a substring of the second. If no such substring exists, you may return the empty string, return any string which isn't a substring of the original string, or throw an exception. If you are returning from a function, you may also return null (or undefined, None, etc.) in this case. If multiple such substrings are tied for the shortest, you may return any one of them.
Strings can consist of any printable ascii characters.
Input given on STDIN will be given with one string on each line. At your request, a single empty line may be added at the end of the input.
This is code golf, so the shortest valid program wins.
SOME TEST CASES
STRING ONE STRING TWO
(Two randomly-generated 80-letter strings)
ALL VALID OUTPUTS:
AD BJ BR CZ DD EE ER EY EY FF FJ FW FZ HE IJ IN IW JC JR JY KL KP KR KV LU MH MI NB NQ OY PK PY QE QF QI RA RB RF RO RV RY RZ SW UE UH UN UR VD VQ VU WB WE WI WU XN XP YI YK YK YM YS YW YX ZB ZJ ZN ZV