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_NoneE: forall j m b2 ofs2,
None = source j m b2 ofs2 ->
forall b1 delta, (b1 < Mem.nextblock m)%positive -> j b1 = Some(b2,delta) ->
~ Mem.perm m b1 (ofs2-delta) Max Nonempty.