<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def copy_to_output_dir(work_dir, output_dir, uuid=None, files=list()):
 """ A list of files to move from work_dir to output_dir. work_dir: str Current working directory output_dir: str Output directory for files to go uuid: str UUID to "stamp" onto output files files: list List of files to iterate through """

    for fname in files:
        if uuid is None:
            shutil.copy(os.path.join(work_dir, fname), os.path.join(output_dir, fname))
        else:
            shutil.copy(os.path.join(work_dir, fname), os.path.join(output_dir, '{}.{}'.format(uuid, fname)))