pub fn to_de_morgan<A: DProp, B: DProp>( (f0, f1): And<Not<A>, Not<B>>) -> Not<Or<A, B>>
(¬a ∧ ¬b) => ¬(a ∨ b).
(¬a ∧ ¬b) => ¬(a ∨ b)