JSFiddle - React, Tailwind, and code Playground

JavaScript

// Lists
const l = {}

// Add Lists to itself
l.l = l

// Lists that list themselves
const ll = {}

// Add Lists to Lists that list themselves
ll.l = l

// Get all the members of L-in-LL
const members = Object.values(ll.l)

// Is L a member of L-in-LL?
console.log(members.includes(l))