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.