I only encountered Plan 9 once, and then only the header inclusion scheme it promotes for it's C code. Which IIRC goes like 'header files must not include other header files hence every source file has to include the headers for all declarations used in that source file and in the headers it includes'. Which means when for any header file you want to include in your source you have to go figure out which headers are needed to pull in all declarations. For every single soure file. I really failed to see how any possible benefits of that would outweigh the cons.
I wouldn't be surprised if plan9 builds are an order of magnitude faster per LoC - Deeply nested includes and cyclic includes waste a huge amount of time.
Avoiding cyclic dependencies might also be a good thing in terms of overall software design.
