theorem Th53:
  U1 c= U2 or U2 in U1
proof
A1: On U1 c= On U2 or On U2 in On U1 by ORDINAL1:16;
A2: U2 = Rank On U2 by Th50;
  U1 = Rank On U1 by Th50;
  hence thesis by A1,A2,CLASSES1:36,37;
end;
