Goal:
  
  A : Type
  P : list A → A → Type
  d : A
  Γ : list A
  a : All_fold P Γ
  p : P Γ d
  b0 : All_fold P (d :: Γ)
  b : sigma (λ index : list A, All_fold P index)
  ============================
   ({| pr1 := d :: Γ; pr2 := b0 |} = b
    → let H := CoreTactics.block in
      NoConfusionHom_All_fold A P (d :: Γ) (All_fold_cons P a p) b0
      → let H0 := CoreTactics.block in All_fold_cons P a p = b0)


DEBUG:Going to execute:
hnf in id
Level 5: 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' := b
of type
constr
id := b
of type
constr of type tacvalue

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' := b
of type
constr
id := b
of type
constr of type tacvalue