Definition kfree n t := rfilter (key n) t.Definition vfree n t := rfilter (var n) t.

(* Specifications: *)
Lemma keyN i n ts : ~~ has (key n) ts -> interp i (kfree n ts) = interp i ts.
Lemma varN i n ts : ~~ has (var n) ts -> interp i (vfree n ts) = interp i ts.