Goal:
  
  A : Type
  P : list A → list A → A → A → Type
  H1' : list A
  H1 : list A
  H : All2_fold P H1' H1
  ============================
   (∀ Γ : list A,
      {| pr1 := Γ; pr2 := Γ |} = {| pr1 := H1'; pr2 := H1 |}
      → let H := CoreTactics.block in
        let H0 := CoreTactics.block in
        All_fold (λ (Γ0 : list A) (d : A), P Γ0 Γ0 d d) Γ)


DEBUG:Going to execute:
hnf in id
Level 4: evaluation returns
hnf in id;
 match type of id with
 | sigma _ =>
     let xn := fresh id "'" in
     destruct id as [xn id]; DepElim.decompose_exists xn id;
      cbv beta iota delta [TODO projection TODO projection] in id, id';
      DepElim.decompose_exists id id'
 | _ => cbv beta iota delta [TODO projection TODO projection] in id, id'
 end
where
id' := H
of type
ident
id := H1
of type
constr of type tacvalue