Show me your immutable flux stores

From the initial post:

Many of my flux stores manage collections and I find myself repeating a lot of boilerplate code for finding / updating / removing objects. In an effort to avoid pulling in something like ampersand-collection, I was hoping to use immutable.js to cut down on some of the duplication but I'm having a hard time wrapping my head around immutability. It actually feels wrong to be using immutable data within the stores since a store's primary job is to mutate data. I must be doing something wrong.

Anyone have some complex stores backed by immutable data structures they could share?