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_splitC: forall (T:Type)(m:mem) (b:block) X (A B:T) (SPLIT: if plt b (Mem.nextblock m) then X = A else X = B) P (HA: Mem.valid_block m b -> X=A -> P) (HB: ~Mem.valid_block m b -> X=B -> P), P.