'Verify that the final combination of on disk files complies with the
erasure-coded diskfile contract.
:param results: files that have been found and accepted
:param frag_index: specifies a specific fragment index .data file
:param frag_prefs: if set, indicates that fragment preferences have
been specified and therefore that a selected fragment is not
required to be durable.
:returns: True if the file combination is compliant, False otherwise'

def _verify_ondisk_files(self, results, frag_index=None, frag_prefs=None, **kwargs):

    if super(ECDiskFileManager, self)._verify_ondisk_files(results, **kwargs):
        have_data_file = (results['data_file'] is not None)
        have_durable = ((results.get('durable_frag_set') is not None) or (have_data_file and (frag_prefs is not None)))
        return (have_data_file == have_durable)
    return False
