theorem Th74:
  \notA\and\notB\imp\not(A\orB) in F
  proof
A1: \not\notA\or\not\notB\imp\not(\notA\and\notB) in F by Th73;
    A\imp\not\notA in F & B\imp\not\notB in F by Th64; then
    A\orB\imp\not\notA\or\not\notB in F by Th59; then
    A\orB\imp\not(\notA\and\notB) in F by A1,Th45; then
    (\notA\and\notB)\imp\not\not(\notA\and\notB) in F &
    \not\not(\notA\and\notB)\imp\not(A\orB) in F by Th64,Th58;
    hence thesis by Th45;
  end;
