Goal:
    A : Type  P : list A → A → Type  b : All_fold P []
  ============================
   (NoConfusionHom_All_fold A P [] (All_fold_nil P) b
    → let H := CoreTactics.block in All_fold_nil P = b)


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end