Function sub_mod 

pub fn sub_mod<T>(a: &T, b: &T, modulo: &T) -> T
where
    T: Ord + for<'x> AddAssign<&'x T> + for<'x> SubAssign<&'x T>,
    for<'x> &'x T: Sub<Output = T> + Neg<Output = T>,

$a - b \bmod n$

Input: $-\text{modulo} \leq a,\, b \leq \text{modulo}$
Output: $-\text{modulo} \leq x \leq \text{modulo}$

use modulo_n_tools::sub_mod;
assert_eq!(sub_mod(&3, &4, &5), -1);
assert_eq!(sub_mod(&2, &-5, &6), 1);
assert_eq!(sub_mod(&-2, &-3, &4), 1);
assert_eq!(sub_mod(&2, &2, &5), 0);