Uint::sub_mod weird behavior (bug?)

I might be wrong but I would expect that any _mod(...) operation would return the Uint in the range of [0, m) (or maybe [-m/2, m/2] for Int) , where m is the modulus.
However this is not the case for sub_mod where the resulting value is way off that range when rhs operand is not in the range.
Not sure is this is expected behavior (if it's not than fixing it would be breaking change), if it is, IMHO it should be documented that arguments must be reduced beforehand.
See example below:

    #[test]
    fn test_sub_mod() {
        let x = U128::from_u128(1);
        let y = U128::from_u128(40);
        let m = U128::from_u128(35);
        let z = x.sub_mod(&y, &m);
        println!("z: {}", u128::from(z));
        // prints  z: 340282366920938463463374607431768211452
    }

for comparison this is how sagemath compute this:

> (1 - 40) % 35
31

Start with the Uint::sub_mod entry point and the test_sub_mod example in the issue. Compare its behavior with the documented modular arithmetic expectations and determine whether unreduced operands are supported; done means the behavior is resolved with a documented contract and an appropriate regression test.