This command is equal to SDIFF, but instead of returning the resulting set, it
is stored in destination.
If destination already exists, it is overwritten (when the difference is nonempty) or deleted (when the difference is empty).
@examples
SADD key1 "a"
SADD key1 "b"
SADD key1 "c"
SADD key2 "c"
SADD key2 "d"
SADD key2 "e"
SDIFFSTORE key key1 key2
SMEMBERS key