diff --git a/src/Init/Data/List/Basic.lean b/src/Init/Data/List/Basic.lean index a09b1dbdab..dbcfb4c3d9 100644 --- a/src/Init/Data/List/Basic.lean +++ b/src/Init/Data/List/Basic.lean @@ -45,6 +45,9 @@ theorem reverseAux_reverseAux (as bs cs : List α) : reverseAux (reverseAux as b | nil => rfl | cons a as ih => simp [reverseAux, ih (a::bs), ih [a]] +@[simp] theorem reverse_reverse (as : List α) : as.reverse.reverse = as := by + simp [reverse]; rw [reverseAux_reverseAux_nil]; rfl + protected def append : List α → List α → List α | [], bs => bs | a::as, bs => a :: List.append as bs