Looks that the code results in that page is broken. I'll fix it tomorrow. It happens because I have to recompile plugin when I change data structure of the language. And the web version can't run because they're loaded as dll/so.

But what I'm doing here is running a validation where the code is something like node.x >= sum(inputs.x), where x is a variable that should always increase while moving down. Or if you think of river elevation for example it has to always decrease while moving downstream. When you have physical laws like that, and you have measurements at locations then you can simply check against that.

Of course you can do that in other systems as well, but Nadi makes it a one liner and intuitive. Some modeling systems don't check and get undefined behavior because of that (just had to debug something like that at work), so Nadi provides a nice way to validate and preprocess data before other systems can use it.