Content files relationship and updates tracking

[ruslan-mogilevskiy]

I have an app.less file that imports other .less files to output a single .css file.
Also, with the following code, I'm skipping all the .less files except app.less:
.AddSetting( WebKeys.Excluded, Config.FromDocument((doc, _) => doc.Destination.Extension == ".less" && doc.Destination.Name != "app.less") )

The problem is that when I start the Statiq live tracking and change other .less files the app.less is not auto-updated so the browser keeps an old app.css file.

Q: How to auto-update another document when some linked document has changed OR how to merge a couple of input or resulting files into a single one that will be saved in the output?
I read many Statiq documentation topics, but there are a lot of them and they are not answering my question.

[ruslan-mogilevskiy]

I've found that if the LESS file is named with the leading underscore then it's excluded from the output, so now I can generate a single CSS file, but the question remains of how to live-update that single CSS when some imported LESS has changed.