theorem ('not' (TVERUM '&&' 'not' A)) => A is ctaut
 proof
   let g;
   set v = VAL g,t = TVERUM;
A1: v.tf = 0 by LTLAXIO1:def 15;
A2: v.A = 0 or v.A = 1 by XBOOLEAN:def 3;
    thus v.(('not' (t '&&' 'not' A)) => A)
    = v.('not' (t '&&' 'not' A)) => v.A by LTLAXIO1:def 15
    .= (v.(t '&&' 'not' A) => v.tf) => v.A by LTLAXIO1:def 15
    .= ((v.t '&' v.('not' A)) => v.tf) => v.A by LTLAXIO1:31
    .= ((v.t '&' (v.A => v.tf)) => v.tf) => v.A by LTLAXIO1:def 15
    .= 1 by A2,A1,Th4;
  end;
