Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  b : All2_dep R' All2_nil
  b0 : sigma
         (λ index : sigma
                      (λ xs : list A, sigma (λ ys : list B, All2 R xs ys)),
            All2_dep R' (pr2 (pr2 index)))
  ============================
   ({|
      pr1 := {| pr1 := []; pr2 := {| pr1 := []; pr2 := All2_nil |} |};
      pr2 := b
    |} = b0
    → let H := CoreTactics.block in
      NoConfusionHom_All2_dep A B R R' [] [] All2_nil All2_dep_nil b
      → let H0 := CoreTactics.block in All2_dep_nil = b)


DEBUG:Going to execute:
simpl in id'