Function median_idx 

fn median_idx<T, F: FnMut(&T, &T) -> bool>(
    v: &[T],
    is_less: &mut F,
    a: usize,
    b: usize,
    c: usize,
) -> usize

🔬This is a nightly-only experimental API. (slice_internals)

returns the index pointing to the median of the 3 elements v[a], v[b] and v[c]