def structures_in_frame(uid)
      raise ArgumentError, "Expected String, got #{uid.class}." unless uid.is_a?(String)
      frame_structures = Array.new
      @structures.each do |s|
        frame_structures << s if s.frame.uid == uid
      end
      return frame_structures
    end
def structures_in_frame(uid)
      raise ArgumentError, "Expected String, got #{uid.class}." unless uid.is_a?(String)
      frame_structures = Array.new
      @structures.each do |s|
        frame_structures << s if s.frame.uid == uid
      end
      return frame_structures
    end
Gives all ROIs/POIs associated with this structure set which belongs to
 the specified Frame of Reference UID.

 @return [Array<ROI, POI>] the matching ROIs/POIs (an empty Array if no structures are matched)