"Union directories are one of the most widely used organizational features of the Plan 9 name space. For instance, the directory/bin is built as a union of /$cpu type/bin (program binaries), /rc/bin (shell scripts), and perhaps more directories provided by the user. This construction makes the shell$PATH variable unnecessary." (plan9.ms)
I did not know that. This might help me play with plan9 more... HolyC is really cool, too. The system Terry Davis built in TempleOS is fascinating. HolyC is JIT, and I think can be rebuilt and executed in place. It's a bastard language for sure; somehow it reminds me of a shell language but actually compiled and statically typed. Why is this not a more commonplace way to design a modern operating system? There's a lot of cruft from Unix left around, it's horrible. Zalgo!
TempleOS is certainly on my shortlist. Any concept of computing that makes it stick in one's head is a path to investigate! Statically typed is a boon for checking code consistency and compiling to the best of the hardware capabilities. Dynamically typed code is a boon for exploration (and, at least with Common Lisp, it can be typed with annotations in a second phase).