Calendar entries currently have a field "access_class" (a choicelist field for the AccessClasses list). This is functionally equivalent to the "hidden" field of comments.Comment. We should unify this feature as a general-purpose model mixin, I'd probably call it Visible or Accessible. it must be in some plugin because it also has a choicelist. For example lino.modlib.system, together with YesNo and Genders.