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

theorem Th6:
  X c= A implies On X = X
proof
  defpred P[object] means $1 in X & $1 is Ordinal;
  assume X c= A;
  then
A1: for x being object holds x in X iff P[x];
A2: for x being object holds x in On X iff P[x] by ORDINAL1:def 9;
  thus thesis from XBOOLE_0:sch 2(A2,A1);
end;
