exMay ∷ Int → Maybe Vertex
vertexMay v | isValidVertex v = Just $ Vertex v
            | otherwise       = Nothing

-- | C
vertexMay ∷ Int → Maybe Vertex
vertexMay v | isValidVertex v = Just $ Vertex v
            | otherwise       = Nothing
vertexMay v | isValidVertex v = Just $ Vertex v
            | otherwise       = Nothing