reserve p,q,r,s,A,B for Element of PL-WFF,
  F,G,H for Subset of PL-WFF,
  k,n for Element of NAT,
  f,f1,f2 for FinSequence of PL-WFF;
reserve M for PLModel;

theorem
  (p => q) => (('not' q) => 'not' p) is tautology
 proof
   let M;
   thus (SAT M).((p => q) => (('not' q) => 'not' p)) =
   (SAT M).(p => q) => (SAT M).(('not' q) => 'not' p) by Def11
   .= ((SAT M).p => (SAT M).q) => (SAT M).(('not' q) => 'not' p) by Def11
   .= ((SAT M).p => (SAT M).q) => ((SAT M).('not' q) => (SAT M).('not' p))
   by Def11
   .= ((SAT M).p => (SAT M).q) => (('not' (SAT M).q) => (SAT M).('not' p))
   by semnot2
   .= ((SAT M).p => (SAT M).q) => (('not' (SAT M).q) => ('not' (SAT M).p))
   by semnot2
   .= 1 by th4;
 end;
