code: function (polygon) {
      let newchild = new PolygonTreeNode()
      newchild.parent = this
      newchild.polygon = polygon
      this.children.push(newchild)
      return newchild
    }
docs: PRIVATE methods from here: add child to a node this should be called whenever the polygon is split a child should be created for every fragment of the split polygon returns the newly created child