t :: Parser (AST' a)
int = do
    n <- integer
    return $
        if n < 0
        then ULiteral $ Int n
        else ULiteral $ Nat n


int :: Parser (AST' a)
int = do
    n <- integer
    return $
        if n < 0
        then ULiteral $ Int n
        else ULiteral $ Nat n
int = do
    n <- integer
    return $
        if n < 0
        then ULiteral $ Int n
        else ULiteral $ Nat n