$BIRCH/local/ is intended as a directory for local customization
of BIRCH. In a sense, it mirrors the organizational structure
of BIRCH. It is NOT overwritten or changed in any way
when a local copy of BIRCH is updated.

$BIRCH/local is analogous to /usr/local in Unix. 
Ideally, nothing is written to /usr/local when the
operating system is upgraded. Everything installed
by the local sysadmin should remain untouched. 
At the same time, the local sysadmin should never
install applications anywhere but /usr/local.
Oh that the creators of RPMs and other automatically
installed software would pay attention to these 
conventions.