Definition  source j m (b2:block) (ofs2:Z) : option(block * Z) :=
if plt 1%positive (Mem.nextblock m)
then sourceN (Pos.pred (Mem.nextblock m)) j m b2 ofs2
else None.

Lemma valid_split: forall m b (A B: Prop)
(SPLIT: (Mem.valid_block m b  -> A) /\ (~Mem.valid_block m b -> B))
(P:Prop)
(HA: Mem.valid_block m b -> A -> P)
(HB: ~Mem.valid_block m b -> B -> P), P.