code: def test_seen(self) -> None:
      d = Document()
      dm = d.models
      assert len(dm) == 0
  
      m1 = Div()
      m2 = Div()
  
      d.add_root(m1)
      d.add_root(m2)
  
      assert not dm.seen(m1.id)
      assert not dm.seen(m2.id)
  
      d.remove_root(m2)
  
      assert not dm.seen(m1.id)
      assert dm.seen(m2.id)
  
      d.remove_root(m1)
  
      assert dm.seen(m1.id)
      assert dm.seen(m2.id)
  
imports: from __future__ import annotations
  import pytest
  import gc
  from unittest.mock import MagicMock, patch
  from bokeh.core.has_props import Local
  from bokeh.core.properties import Instance, Nullable
  from bokeh.core.types import ID
  from bokeh.document import Document
  from bokeh.model import Model
  from bokeh.models import Div, Row
  import bokeh.document.models
label_desc: Use image node_id 13 for calling the TestDocumentModelManager obj's underlying member method code with example usage: obj.test_seen() without return types