What is the Scheme of operating systems?
To answer this, we need to know what an operating system should be at its baseline.
Is there an operating system that is suitable for both education and practical use?
The operating system which the Viewpoints Research Institute was building is probably the closest:
http://vpri.org
Probably Minix, but it seems to have been abandoned by its authors.
No UNIX clone qualifies.
>>2
How many millions of lines is OpenBSD?