Streams.

Impressed by lectures of Gerald Jay Sussmann and Harold Abelson, I decided to put the streams into the rendering engine to simplify it and to add lazy calculations. I'm too "imperative" and it takes some time and pain to move to functional style.

One may say "First implement it and then play", but I desagree. Features are just a bunch of code which will prevent me from doing any changes. So now is the most correct time.