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 source_SomeE: forall j m (b2:block) (ofs2:Z) p, Some p = source j m b2 ofs2 -> exists b1, exists delta, exists ofs1, p = (b1,ofs1) /\ (b1 < Mem.nextblock m)%positive /\ j b1 = Some(b2,delta) /\ Mem.perm m b1 ofs1 Max Nonempty /\ ofs2=ofs1+delta.