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I know exactly how it was done, I am just amazed that it was done. It's a great idea that I have never come up with, nor would I be able to.

Yes, it's a great idea.

A similar idea is used to trace what 'make install' does so that stuff can be uninstalled later - or made into a package (rpm, dpkg etc)

I forget what that tool is called.

Also similar in concept is the tup build system which monitors rule commands run to gain knowledge of additional dependencies.