3      ~(A | B) => ~A & ~B;  by Lemma deMorganNotOr
4.tut:43.7-43.16 Error:
Unjustified line ~(A & B); ~(~A | ~B); ~(A | B) => ~A & ~B  |-  ~~A & ~~B
Assuming this line, checking remainder...
  4      ~~A & ~~B;            UNJUSTIFIED