theorem Th29:
  X c= Y implies still_not-bound_in X c= still_not-bound_in Y
proof
  set A = {still_not-bound_in p : p in X};
  assume
A1: X c= Y;
    let a be object;
    assume a in still_not-bound_in X;
    then consider b such that
A2: a in b and
A3: b in A by TARSKI:def 4;
    ex p st ( b = still_not-bound_in p)&( p in X) by A3;
    then b in {still_not-bound_in q : q in Y} by A1;
    hence a in still_not-bound_in Y by A2,TARSKI:def 4;
end;
