clarified NEWS: document_files are officially required since Isabelle2014, but the absence was tolerated as legacy feature;
@PhdThesis{Ballarin:1999,
author = {Clemens Ballarin},
title = {Computer Algebra and Theorem Proving},
school = {University of Cambridge},
year = 1999,
note = {Also Computer Laboratory Technical Report number 473.}
}
@Book{Jacobson:1985,
author = {Nathan Jacobson},
title = {Basic Algebra I},
publisher = {Freeman},
year = 1985
}
@Article{Kammueller-Paulson:1999,
author = {Florian Kamm\"uller and L. C. Paulson},
title = {A Formal Proof of Sylow's theorem: An Experiment in
Abstract Algebra with {Isabelle HOL}},
journal = {J. Automated Reasoning},
year = 1999,
number = 23,
pages = {235--264}
}