So I've merged it for now. I wonder though, what exactly is the purpose of it?

Is it mostly to just say "one or many"? Or do we want immutability? Does it really make sense to allow for array access? Wouldn't it potentially be better to have a more opaque implementation that could potentially be lazy, so that e.g. seq.filter(...).slice(x, y).map(...).filter(...).map(...) really just constructs one sequence, and it only does it when the iterator on said sequence is created?

I'm just throwing around ideas. Maybe you could share your experience about how you've been using this @kevinresol ?