Goal:
    A : Type  P : list A → A → Type
  ============================
   (let H := CoreTactics.block in All_fold_nil P = All_fold_nil P)


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end