interacsion

use core::cell::RefCell;
use std::collections::{BTreeMap, VecDeque};
use std::rc::Rc;

pub trait GroupBy<Item> {
    fn group(self) -> impl Iterator<Item = (Item, impl Iterator<Item = Item>)>
    where
        Item: PartialEq + Copy;

    fn group_by<Key, KeyFn>(