code: def test_opts_clear_cleans_unused_tree(self):
      ExampleElement([]).opts(style_opt1="A").opts.clear()
      custom_options = Store._custom_options["backend_1"]
      self.assertEqual(len(custom_options), 0)
  
imports: import gc
  from holoviews.core.element import Element
  from holoviews.core.options import Keywords, Options, OptionTree, Store
  from holoviews.core.spaces import HoloMap
  from ..utils import LoggingComparisonTestCase
label_desc: Use image node_id 6 for calling the TestOptionsCleanup obj's underlying member method code with example usage: obj.test_opts_clear_cleans_unused_tree() without return types