Arguments retainer_part_nonreadable: clear implicits.

Lemma comp_parts_join:
forall L R (HC: Share.comp L = R) a b c,
join (Share.glb L a) (Share.glb L b) (Share.glb L c) ->
join (Share.glb R a) (Share.glb R b) (Share.glb R c) ->
join a b c.