[jeremyjh]

I think you're talking about two different things. When they say concurrency is hidden away, I think they mean the programmer isn't dealing directly with locks/mutexes. They are also lying, since async functionality like `handle_cast` is built into the `gen_server` behaviour and every caller is calling either `gen_server:cast` or `gen_server:call` depending on whether or not it is going to block on a response. Whenever you invoke async behaviour concurrency will invade all the domain logic and can't be ignored.