// Definition 38 For two terms T1 and T2 , their product (T1 × T2 ) is a compound term defined by
// ((S1 × S2 ) → (P1 × P2 )) ≡ ((S1 → P1 ) ∧ (S2 → P2 )).
<(*,S1, S2 ) --> (*,P1, P2 )>.
<S1 --> P1>?
<S2 --> P2>?
(&&,<S1 --> P1>, <S2 --> P2>)?