code: String assertion() {
          String assertion = assertionForEmailAndSite(email, getRemoteURL());
          if (assertion == null) {
              Log.w(TAG, "PersonaAuthorizer<%s> no assertion found for: %s", email, getRemoteURL());
              return null;
          }
          Map<String, Object> result = parseAssertion(assertion);
          if (result == null || isAssertionExpired(result)) {
              Log.w(TAG, "PersonaAuthorizer<%s> assertion invalid or expired: %s", email, assertion);
              return null;
          }
          return assertion;
      }
docs: /*package