pub trait SecondaryIndexEntry: Debug {
// Required methods
fn insert_if_not_exists(&self, key: &Pubkey, inner_keys_count: &AtomicU64);
fn remove_inner_key(&self, key: &Pubkey) -> bool;
fn is_empty(&self) -> bool;
fn keys(&self) -> Vec<Pubkey> ⓘ;
fn len(&self) -> usize;
}