def add_poi(poi)
      raise ArgumentError, "Invalid argument 'poi'. Expected POI, got #{poi.class}." unless poi.is_a?(POI)
      @structures << poi unless @structures.include?(poi)
    end
def add_poi(poi)
      raise ArgumentError, "Invalid argument 'poi'. Expected POI, got #{poi.class}." unless poi.is_a?(POI)
      @structures << poi unless @structures.include?(poi)
    end
Adds a POI instance to this StructureSet.

 @param [POI] poi a poi instance to be associated with this structure set