input: Write a python function to check if a user is already registered by email.
  assert check_user_registered('test@example.com') == False
  assert check_user_registered('existing_user@example.com') == True
  assert check_user_registered('new_user@example.com') == False
output: def check_user_registered(email):
      registered_emails = {'existing_user@example.com'}  # Example of registered emails
      return email in registered_emails