Goal:
    A : Type  B : Type  R : nat → A → B → Type  n : nat
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2i
      (λ (i : nat) (x : A) (y : B),
         (if i <? n then False else nth_error [] (i - n) = Some x) × R i x y)
      n [] [])


DEBUG:Going to execute:
match goal with
           | hyp:context [ CoreTactics.block ]
             |- _ =>
                 cbn beta in hyp; eqns_specialize_eqs_block hyp;
                  cbn[eq_rect_r eq_rect] in hyp
           end