reserve fi,psi for Ordinal-Sequence,
  A,A1,B,C,D for Ordinal,
  X,Y for set,
  x,y for object;

theorem
  A in B implies B = A+^(B-^A)
proof
  assume A in B;
  then A c= B by ORDINAL1:def 2;
  hence thesis by Def5;
end;
