Function retool_begin 

pub fn retool_begin<T, V, C: IncDecCpCmp<T, V>, R: GetBeginEnd<T>>(
    res: (T, T),
    src: &[R],
    step: &V,
    t: &C,
) -> (T, T)

Computes the final value from a result of crate::reduce_back.