Function is_ring 

pub fn is_ring<T, Add, Mul>(elements: &[T], add: Add, multiply: Mul) -> bool
where
    T: Copy + PartialEq,
    Add: Fn(T, T) -> T + Copy,
    Mul: Fn(T, T) -> T + Copy,

Commonly used facade items. Returns whether elements with add and multiply form a ring.