Found in 1 comment on Hacker News
jasomill · 2012-05-25 · Original thread
Of which your source file is an excellent example. Run, IIRC,

    weave mf.web
to generate TeX sources for a camera-ready, exhaustively commented version of the source code that has even been published in hardcover[1].

[1] http://www.amazon.com/Computers-Typesetting-Volume-Metafont-...

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