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_eq: forall sh1 (rsh1: readable_share sh1) sh2 (nsh2: ~readable_share sh2) sh3 (rsh3: readable_share sh3), join sh1 sh2 sh3 -> readable_part rsh1 = readable_part rsh3.