even on plan 9,
mk clean clears out the object files that are now installed in the libraries,
but leaves the libraries.  (some other targets are sometimes left behind, not just libraries.)
mk nuke removes the target libraries (and sometimes other targets) as well.
the distinction is helpful when working with the compilers, and perhaps upas.
mk pristine might be a better name, since nukes leave quite a mess, but
it's harder to type.