Add another TODO

theories/examples/list_helper.v

@@ -4,7 +4,7 @@ From orc11 Require Import base.
 From stdpp Require Import options.
 
 (* TODO: upstream to stdpp *)
-
+(* TODO: cleanup unused lemmas *)
 Section prefixes.
 Context {A: Type}.
 Implicit Types (B: Type) (l: list A) (x : A).