<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validate_iss(claims, issuer=None):
 """Validates that the 'iss' claim is valid. The "iss" (issuer) claim identifies the principal that issued the JWT. The processing of this claim is generally application specific. The "iss" value is a case-sensitive string containing a StringOrURI value. Use of this claim is OPTIONAL. Args: claims (dict):
 The claims dictionary to validate. issuer (str or iterable):
 Acceptable value(s) for the issuer that signed the token. """


    if issuer is not None:
        if isinstance(issuer, string_types):
            issuer = (issuer,)
        if claims.get('iss') not in issuer:
            raise JWTClaimsError('Invalid issuer')