code: def test_get_placeholders(self):
      placeholders = prompt_utils.get_placeholders(
          "today {hello} world"
      )
      self.assertEqual(frozenset({"hello"}), placeholders)
  
      placeholders = prompt_utils.get_placeholders("{hello} {world}")
      self.assertEqual(frozenset({"hello", "world"}), placeholders)
  
      placeholders = prompt_utils.get_placeholders("{hello} {hello}")
      self.assertEqual(frozenset({"hello"}), placeholders)
  
imports: from __future__ import annotations
  from absl.testing import absltest
  from google.generativeai.notebook.lib import prompt_utils
label_desc: Use image node_id 2 for calling the PromptUtilsTest obj's underlying member method code with example usage: obj.test_get_placeholders() without return types