Hello, seems I really don't understand much presently.
The p9p install(1) says: ---- Once the system is built for the first time, it can be maintained and rebuilt using mk(1). To rebuild individual commands or libraries, run mk install and mk clean in the appropriate source directory (see src(1)). ---- I use hg. So from time to time I do 'hg pull -u'. Now, it seems to be always safe to run ./INSTALL from the top directory. This, however, seems to build everything over again, probably unnecessarily. I thought I'd call sth like 'mk install' from the top directory to build only new/changed stuff. However there is no mkfile there. How should I understand the cited? Thanks! Ruda