theorem naa:
  F |- ('not' A => A) => A
  proof
    'not' A => 'not' A => (('not' A => A)=>A) in PL_axioms by withplax;then
A1: F |- ('not' A => 'not' A) => (('not' A => A)=>A) by th42;
    F |- 'not' A => 'not' A by thaa;
    hence thesis by A1,th43;
  end;
