Arguments retainer_part_nonreadable: clear implicits.

Lemma left_right_join:
forall a b c,
join (Share.glb Share.Lsh a) (Share.glb Share.Lsh b) (Share.glb Share.Lsh c) ->
join (Share.glb Share.Rsh a) (Share.glb Share.Rsh b) (Share.glb Share.Rsh c) ->
join a b c.