concat vs union

test.js

const {OrderedSet} = Immutable;

const x = OrderedSet([]);
const y = OrderedSet([2, 3, 4, 5, 6]);

const z = x.concat(y);

console.log(z === y);

// This will log `false`.
//
// However if we call `union` instead, it will log `true`.
//
// I think both results make sense for their context, `concat` coming
// from Lists and `union` coming from Sets. I think we want to be 
// calling `concat` most of the time here.