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 semnot2:
  (SAT M).('not' p) = 'not' (SAT M).p
  proof
    thus (SAT M).('not' p) = (SAT M).p => (SAT M).FaLSUM by Def11
    .= (SAT M).p => FALSE by Def11
    .= 'not' (SAT M).p;
  end;
