input: Write a python function to respond to specific greetings and farewells.
  assert respond('hi') in ['Hi there!', 'Hello there!', 'Greetings!']
  assert respond('bye') in ['See you later!', 'Goodbye!', 'Bye!']
  assert respond('hello') in ['Hi there!', 'Hello there!', 'Greetings!']
  assert respond('Later') in ['See you later!', 'Goodbye!', 'Bye!']
output: def respond(greeting):
      greeting = greeting.lower()
      if greeting in ['hi', 'hello']:
          return 'Hi there!'
      elif greeting in ['bye', 'later']:
          return 'See you later!'
      else:
          return 'I don’t understand!'