theorem Th26:
  still_not-bound_in {p} = still_not-bound_in p
proof
A1: now
    let a be object;
    assume a in still_not-bound_in {p};
    then consider b such that
A2: a in b & b in {still_not-bound_in q : q in {p}} by TARSKI:def 4;
    ex q st ( b = still_not-bound_in q)&( q in {p}) by A2;
    hence a in still_not-bound_in p by A2,TARSKI:def 1;
  end;
  now
    let a be object such that
A3: a in still_not-bound_in p;
    set b = still_not-bound_in p;
    p in {p} by TARSKI:def 1;
    then b in {still_not-bound_in q : q in {p}};
    hence a in still_not-bound_in {p} by A3,TARSKI:def 4;
  end;
  hence thesis by A1,TARSKI:2;
end;
