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


DEBUG:Going to execute:
CoreTactics.block_goal
Level 8: evaluation returns
idtac of type tacvalue

Level 6: evaluation returns
idtac of type tacvalue

Level 5: evaluation returns
idtac of type tacvalue