when /n/src9 is an alias for / (bind / /n/src9), then the bind
from /sys/lib/dist/usr on /n/src9/usr would also overbind
/usr and potentially make the build target file inaccessible.
the solution is to run aux/stub -d /n/src9/usr.
this works as aux/stub mounts itself on /n/src9, so the bind
for /n/src9/usr will not override /usr in the root filesystem
but the stub directory from aux/stub.
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