theorem Th21:
  p in X implies X |- p
proof
  assume
A1: p in X;
  now
    let a be object;
    assume a in rng <*p*>;
    then a in {p} by FINSEQ_1:38;
    hence a in X by A1,TARSKI:def 1;
  end;
  then
A2: rng <*p*> c= X;
  |- <*>CQC-WFF(Al)^<*p*>^<*p*> by CALCUL_2:21;
  then |- <*p*>^<*p*> by FINSEQ_1:34;
  hence thesis by A2,HENMODEL:def 1;
end;
