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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def _build_gecos(gecos_dict):
    '''
    Accepts a dictionary entry containing GECOS field names and their values,
    and returns a full GECOS comment string, to be used with usermod.
    '''
    return '{0},{1},{2},{3}'.format(gecos_dict.get('fullname', ''),
                                    gecos_dict.get('roomnumber', ''),
                                    gecos_dict.get('workphone', ''),
                                    gecos_dict.get('homephone', ''))