reserve A,A1,A2,B,C,D for Ordinal,
  X,Y for set,
  x,y,a,b,c for object,
  L,L1,L2,L3 for Sequence,
  f for Function;

theorem Th1:
  A c= B iff succ A c= succ B
proof
  A c= B iff A in succ B by ORDINAL1:22;
  hence thesis by ORDINAL1:21;
end;
