Function bisection::insort_left_by

pub fn insort_left_by<T, F>(a: &mut Vec<T>, x: T, f: F) where
    T: Ord,
    F: FnMut(&T, &T) -> Ordering, 

Insert x in a, keeping it sorted, assuming a is sorted, according to a comparator function.

The comparator function should implement an order consistent with the sort order of the underlying slice.

If a contains x, insert it just before the leftmost occurence of x.