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 semdis2:
  (SAT M).(A 'or' B) = (SAT M).A 'or' (SAT M).B
  proof
    thus (SAT M).(A 'or' B) = (SAT M).('not' A) => (SAT M).B by Def11
    .= (SAT M).A 'or' (SAT M).B by semnot2;
  end;
