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.