<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def select(files, start, stop):
 """ Helper function for handling start and stop indices """

    if start or stop:
        if start is None:
            start = 0
        if stop is None:
            stop = len(files)
        files = files[start:stop]
    return files