Goal:
    A : Type  P : list A → A → Type  b : All_fold P []
  ============================
   (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:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')