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

theorem
  A c= B+^(A-^B)
proof
  now
    per cases;
    suppose
      B c= A;
      hence thesis by Def5;
    end;
    suppose
A1:   not B c= A;
      then A-^B = {} by Def5;
      hence thesis by A1,ORDINAL2:27;
    end;
  end;
  hence thesis;
end;
