We've got the ability to append to lists,
I'm using this to try to read all of the contents of multiple chests, and I've been unable to find/create functions that will work to concatenate multiple reader item lists together.

This functionality might be useful when creating a list of items from one or more inventory that satisfies a predicate.

Is this something already do-able and I'm just missing it?

Oh, that doesn't exist yet?
I'll implement that for next release, as that is indeed quite essential.

Implemented in f0cebea