Goal:
  
  A : Type
  P : list A → A → Type
  b : All_fold P []
  b0 := {| pr1 := []; pr2 := b |}
     : sigma (λ index : list A, All_fold P index)
  ============================
   (let H := CoreTactics.block in
    NoConfusionHom_All_fold A P [] (All_fold_nil P) b
    → let H0 := CoreTactics.block in All_fold_nil P = b)


DEBUG:Going to execute:
generalize (eq_refl : v = id'); clearbody id'; simpl in id';
            cont id id' id v