proper support for "isabelle update -D DIR": avoid accidental exclusion of select_dirs (amending e5dafe9e120f);
@string{CUP="Cambridge University Press"}
@string{LNCS="Lect.\ Notes in Comp.\ Sci."}
@string{Springer="Springer-Verlag"}
@manual{Main,author={Tobias Nipkow},title={What's in Main},
note={\url{https://isabelle.in.tum.de/doc/main.pdf}}}
@manual{ProgProve,author={Tobias Nipkow},
title={Programming and Proving in Isabelle/HOL},
note={\url{https://isabelle.in.tum.de/doc/prog-prove.pdf}}}
@manual{IsarRef,author={Makarius Wenzel},
title={The Isabelle/Isar Reference Manual},
note={\url{https://isabelle.in.tum.de/doc/isar-ref.pdf}}}