proper support for "isabelle update -D DIR": avoid accidental exclusion of select_dirs (amending e5dafe9e120f);
@book{card-book,
title = {Introduction to {C}ardinal {A}rithmetic},
author = {M. Holz and K. Steffens and E. Weitz},
publisher = "Birkh{\"{a}}user",
year = 1999,
}
@article{taylor-ordinals,
author = {Paul Taylor},
title = {Intuitionistic Sets and Ordinals},
journal = {J. Symb. Log.},
volume = {61},
number = {3},
year = {1996},
pages = {705-744},
bibsource = {DBLP, http://dblp.uni-trier.de}
}