addPrvKeys :: PrvKey -> Word256 -> Maybe PrvKey
addPrvKeys key i
    | isPrvKeyU key = error "Add: HDW only supports compressed formats"
    | toInteger i < curveN =
        let r = (prvKeyFieldN key) + (fromIntegral i :: FieldN) 
            in makePrvKey $ toInteger r
    | otherwise = Nothing
addPrvKeys :: PrvKey -> Word256 -> Maybe PrvKey
addPrvKeys key i
    | isPrvKeyU key = error "Add: HDW only supports compressed formats"
    | toInteger i < curveN =
        let r = (prvKeyFieldN key) + (fromIntegral i :: FieldN) 
            in makePrvKey $ toInteger r
    | otherwise = Nothing
addPrvKeys key i
    | isPrvKeyU key = error "Add: HDW only supports compressed formats"
    | toInteger i < curveN =
        let r = (prvKeyFieldN key) + (fromIntegral i :: FieldN) 
            in makePrvKey $ toInteger r
    | otherwise = Nothing