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_map_self (A : Type) (f : positive -> A -> A) t :
(forall b a, t ! b = Some a -> f b a = a) ->
PTree.map f t = t.