workaround for old macOS versions, after change of Let's Encrypt root certificate --- see also https://letsencrypt.org/docs/dst-root-ca-x3-expiration-september-2021 --- Java/Scala Isabelle_System.download() works, but curl doens't;
src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools