Ah, I was looking in the "Frequently Suggested Ideas" viewtopic.php?f=80&t=4806 for this.

This has been discussed in length several times, and the devs themselves have said that this is not a bug with the underground belts, and that its the "Proper way" to block one side of a belt.

Haven't find it in the "Ideas and Suggestions" forum.
Could You please give a link to that dev's post ...

Some time ago (almost 2 years) there existed a mod called Belt Blocker ( https://forums.factorio.com/viewtopic.php?f=87&t=6227 ) which added a way to stop just one lane of a belt, but the mod isn't compatible with the latest version of the game.

Currently there is an alternative way to do this, by ...

About Lua hacking:
just as an idea: try Lua mailing list.

small bug: chemical plant production is affected by assembler level setting.

This is absolutely amazing! Regrettably I'm feeling really stupid, but I can't figure out what some of the numbers mean.
I'm trying to build a perfect factorio from the very beginning, so i'm asking myself (being a noob), what is the best efficient way to produce green circuits. At rate of 4 i/s I ...