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 dec_split: forall {C} (dec: C \/ ~ C) (A B: Prop) (SPLIT: (C -> A) /\ (~C -> B)) (P:Prop) (HA: C -> A -> P) (HB: ~C -> B -> P), P.