usr/src/sys/miscfs/union/README

If you plan on using union mounts, then you should consider replacing
"libc/gen/opendir.c" in the C library with the file "libc.opendir.c"
in this directory.  The replacement version of opendir() automatically
removes duplicate names when a union stack is encountered.  You will
then need to rebuild the C library and all commands.

@(#)README      8.1 (Berkeley) %G%