Corollary atomic_entries_join_nil : forall sh1 sh2 sh entries ghosts
  (Hjoin : sepalg.join sh1 sh2 sh) (Hlen : Zlength entries = size) (Hleng : Zlength ghosts = size)
  (Hsh1 : readable_share sh1) (Hsh2 : readable_share sh2),
  atomic_entries sh1 entries ghosts empty_hists * atomic_entries sh2 entries ghosts empty_hists =
  atomic_entries sh entries ghosts empty_hists.

Lemma join_hists_length : forall h1 h2, Zlength h1 = Zlength h2 -> Zlength (join_hists h1 h2) = Zlength h1.