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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 verify_token(id_token, request, audience=None, certs_url=_GOOGLE_OAUTH2_CERTS_URL):
 """Verifies an ID token and returns the decoded token. Args: id_token (Union[str, bytes]):
 The encoded token. request (google.auth.transport.Request):
 The object used to make HTTP requests. audience (str):
 The audience that this token is intended for. If None then the audience is not verified. certs_url (str):
 The URL that specifies the certificates to use to verify the token. This URL should return JSON in the format of ``{'key id': 'x509 certificate'}``. Returns: Mapping[str, Any]: The decoded token. """

    certs = _fetch_certs(request, certs_url)

    return jwt.decode(id_token, certs=certs, audience=audience)