[karmakaze]

What is this 'core stack' you speak of, is it special in some way?

I once did an all day on-site pairing interview with Pivotal. The morning was Swift/iOS and afternoon was Java back-end. I didn't have experience in either except some Java desktop GUI from a while back. It was all fine, they wanted to see how I think, what code paths I think of, what tests I choose for coverage. The actual syntax of what's being written wasn't the main point. That translates quickly on the job from experience doing similar tasks in other environments. If on the other hand, you've never written a test, or discussed code with a colleague those are not as easy to pick up from somewhere else.