A user-interaction utility.  This is an adaptation of a very old (late
1980s) utility.  The old one had an API that is not portable enough -
it basically assumed that all pointer types are effectively identical.
It originated as VAX-specific FORTRAN code, later rewritten in C and
adapted to Suns as well - you can find that version as the
libCtree-20011025 branch in this repo.  This is a ground-up redesign.