theorem semequ2:
  (SAT M).(A <=> B) = (SAT M).A <=> (SAT M).B
  proof
    thus (SAT M).(A <=> B) = (SAT M).(A => B) '&' (SAT M).(B => A) by semcon2
    .= ((SAT M).A => (SAT M).B) '&' (SAT M).(B => A) by Def11
    .= (SAT M).A <=> (SAT M).B by Def11;
    end;
