<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _default_output_prefix(filenames):
 """ Return a default filename prefix for output files based on the input files. :param filenames: The input filename(s):
 :type filenames: str or list of str :returns: The extensionless common prefix of the input filenames: :rtype: str """


    if isinstance(filenames, (str, )):
        filenames = [filenames]
    common_prefix, ext = os.path.splitext(os.path.commonprefix(
        map(os.path.basename, filenames)))
    common_prefix = common_prefix.rstrip("_-")
    return common_prefix if len(common_prefix) > 0 else "sick"