theorem Th23: 'not' TVERUM => p is ctaut
  proof
    let g;
    set v = VAL g;
A1: v.tf = 0 by LTLAXIO1:def 15;
    thus v.('not' TVERUM => p) = v.('not' TVERUM) => v.p by LTLAXIO1:def 15
    .= v.TVERUM => v.tf => v.p by LTLAXIO1:def 15
    .= TRUE => v.tf => v.p by Th4
    .= 1 by A1;
  end;
