Function reactive_index 

pub fn reactive_index<K, S, V>(
    initial: Vec<IndexRow<K, S, V>>,
    options: ReactiveIndexOptions,
) -> ReactiveIndex<K, S, V>
where
    K: Clone + Ord + Debug + 'static,
    S: Clone + Ord + 'static,
    V: Clone + 'static,

Creates or computes reactive_index.