f.hs

    getVrf :: [IfIP] -> Comp1 (Maybe Vrf)
    getVrf ips = do
      vrfs <- traverse findVrf (toListOf opt ips)
      case nub vrfs of
        [] -> pure Nothing
        [x] -> pure (Just x)
        _xs -> Nothing <$ configErr (vsep [ "Multiple VRFs present:"
                                          , indent 4 (hyphenList (pretty . vrfName <$> vrfs))
                                          ])