theorem Th86:
  Subformulae(P!V) = { P!V }
proof
  thus Subformulae(P!V) c= { P!V }
  proof
    let a be object;
    assume a in Subformulae(P!V);
    then consider F such that
A1: F = a and
A2: F is_subformula_of P!V by Def22;
    F = P!V by A2,Th80;
    hence thesis by A1,TARSKI:def 1;
  end;
  let a be object;
  assume a in { P!V };
  then a = P!V by TARSKI:def 1;
  hence thesis by Def22;
end;
