Definition xkeys (A: Type) (m: t A) (i: positive) :=
List.map (@fst positive A) (xelements m i nil).
Remark xkeys_leaf:
forall A i, xkeys (@Leaf A) i = nil.
Remark xkeys_node:
forall A (m1: t A) o (m2: t A) i,
xkeys (Node m1 o m2) i =
xkeys m1 (xO i)
++ match o with None => nil | Some v => prev i :: nil end
++ xkeys m2 (xI i).

Lemma in_xkeys:
forall (A: Type) (m: t A) (i k: positive),
In k (xkeys m i) ->
(exists j, k = prev (prev_append j i)).