Function partition_equal 

fn partition_equal<T, F>(v: &mut [T], pivot: usize, is_less: &F) -> usize
where
    F: Fn(&T, &T) -> bool,

Partitions v into elements equal to v[pivot] followed by elements greater than v[pivot].

Returns the number of elements equal to the pivot. It is assumed that v does not contain elements smaller than the pivot.