proper support for "isabelle update -D DIR": avoid accidental exclusion of select_dirs (amending e5dafe9e120f);
@string{CUCL="Computer Laboratory, University of Cambridge"}
@TechReport{camilleri92,
author = {J. Camilleri and T. F. Melham},
title = {Reasoning with Inductively Defined Relations in the
{HOL} Theorem Prover},
institution = CUCL,
year = 1992,
number = 265,
month = Aug}