Instance Join_rshare: Join rshare :=
  fun a b c => join (proj1_sig a) (proj1_sig b) (proj1_sig c).Instance Perm_rshare: Perm_alg rshare.

Lemma join_readable_part: forall sh1 (rsh1: readable_share sh1) sh2 (rsh2: readable_share sh2) sh3 (rsh3: readable_share sh3), join sh1 sh2 sh3 -> join (readable_part rsh1) (readable_part rsh2)(readable_part rsh3).