def _latest_common_snapshot(some, others):
 others_set = set(others)
 for snapshot in reversed(some):
  if (snapshot in others_set):
   return snapshot
 return None

def _latest_common_snapshot ( some , others ) : others_set = set ( others ) for snapshot in reversed ( some ) : if ( snapshot in others_set ) : return snapshot return None
pick the most recent snapshot that is common to two snapshot lists .