code: public Collection<Workitem> workitems(WorkitemFilter filter) {
          return get(Workitem.class, (filter != null) ? filter : new WorkitemFilter());
      }
docs: Get workitems (stories, defects, tasks, and tests) filtered by the
  criteria specified in the passed in filter.
  
  @param filter Limit the items returned. If null, then all items returned.
  @return Collection of items as specified in the filter.