Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  a : A
  Γ : list A
  d' : A
  Γ'0 : list A
  Hrel : All2_fold P Γ Γ'0
  p : P Γ Γ'0 a d'
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    ∑ d'0 : A,
      nth_error (d' :: Γ'0) 0 = Some d'0
      × (let Γs := skipn 1 (a :: Γ) in
         let Γs' := skipn 1 (d' :: Γ'0) in
         All2_fold P Γs Γs' × P Γs Γs' a d'0))


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