reserve X,Y for set, x,y,z for object, i,j,n for natural number;
reserve
  n for non empty Nat,
  S for non empty non void n PC-correct PCLangSignature,
  L for language MSAlgebra over S,
  F for PC-theory of L,
  A,B,C,D for Formula of L;

theorem Th73:
  \notA\or\notB\imp\not(A\andB) in F
  proof
    A\andB\impA in F & A\andB\impB in F by Def38; then
A1: \notA\imp\not(A\andB) in F & \notB\imp\not(A\andB) in F by Th58;
    \notA\imp\not(A\andB)\imp(\notB\imp\not(A\andB)\imp
    (\notA\or\notB\imp\not(A\andB))) in F by Def38; then
    \notB\imp\not(A\andB)\imp(\notA\or\notB\imp\not(A\andB)) in F by A1,Def38;
    hence \notA\or\notB\imp\not(A\andB) in F by A1,Def38;
  end;
