example : Array.eraseReps #[1, 3, 2, 2, 2, 3, 5] = #[1, 3, 2, 3, 5] := rfl
example : List.eraseReps   [1, 3, 2, 2, 2, 3, 5] =  [1, 3, 2, 3, 5] := rfl