class_docstr: Creates ShapelyAnnotation (as shapely MultiPolygon).
  Can convert this instance annotation to various formats.
code: def multipolygon(self, multipolygon: MultiPolygon):
      self.__multipolygon = multipolygon
      # calculate areas of all polygons
      area = 0
      for shapely_polygon in multipolygon.geoms:
          area += shapely_polygon.area
      # set instance area
      self.__area = area
  
imports: from typing import List
  from shapely.geometry import CAP_STYLE, JOIN_STYLE, MultiPolygon, Polygon, box
label_desc: Use image node_id 6 for calling the ShapelyAnnotation obj's underlying member method code with example usage: obj.multipolygon(multipolygon) without return types