sBin :: (String -> String -> String) -> [Const] -> Maybe Const
sBin op [Str x, Str y] = Just $ Str (op x y)
sBin :: (String -> String -> String) -> [Const] -> Maybe Const
sBin op [Str x, Str y] = Just $ Str (op x y)
sBin op [Str x, Str y] = Just $ Str (op x y)