Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  fn : ∀ x : A, (∀ y : C, Q y → P y x) → B
  x : A
  xs : list A
  h : ∀ y : C, Q y → ∥ All (P y) (x :: xs) ∥
  H : ∀ (n : nat) (x0 : A),
        nth_error xs n = Some x0
        → ∃ D : ∀ y : C, Q y → P y x0,
            nth_error (map_All fn xs (map_All_obligation_2 x xs h)) n =
            Some (fn x0 D)
  x0 : A
  H2 : option A
  H1 : Some x = H2
  ============================
   (Some x0 = H2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      ∃ D : ∀ y : C, Q y → P y x0,
        nth_error (map_All fn (x :: xs) h) 0 = Some (fn x0 D))


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' := H1
of type
constr
id := H2
of type
constr of type tacvalue

Level 3: 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' := H1
of type
constr
id := H2
of type
constr of type tacvalue