NOT thread-safe!
    return stems or stem# where # is the smallest
    positive integer for which the path does not exist.
    useful for temp dirs where the client code wants an
    obvious ordering.

def get_unique_filepath(stem):
    """NOT thread-safe!
    return stems or stem# where # is the smallest
    positive integer for which the path does not exist.
    useful for temp dirs where the client code wants an
    obvious ordering.
    """
    fp = stem
    if os.path.exists(stem):
        n = 1
        fp = stem + str(n)
        while os.path.exists(fp):
            n += 1
            fp = stem + str(n)
    return fp