Provided two arguments, seq (a sequence of digits 0-9) and sub (a sub-sequence of digits 0-9), return a new sequence containing every digit, in order, that comes immediately before or after every appearance of sub in seq. If a digit is shared by two appearances, it should be included twice in the output. Appearances may be at the beginning and/or end of seq, in which case only existing digits should be included. Assume that sub.length() > 0 and that there are no overlapping appearances.