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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def GetDefaultToken(token):
 """Returns the provided token or the default token. Args: token: A token or None. Raises: access_control.UnauthorizedAccess: no token was provided. """

  if token is None:
    token = default_token

  if not isinstance(token, access_control.ACLToken):
    raise access_control.UnauthorizedAccess(
        "Token is not properly specified. It should be an "
        "instance of grr.lib.access_control.ACLToken()")

  return token