Definition same_except_cur (m m' : Mem.mem) :=
Mem.mem_contents m = Mem.mem_contents m' /\
max_access_at m = max_access_at m' /\
Mem.nextblock m = Mem.nextblock m'.

Lemma PTree_xmap_self A f (m : PTree.t A) i :
(forall p a, m ! p = Some a -> f (PTree.prev_append i p) a = a) ->
PTree.xmap f m i = m.