discontinue somewhat pointless thread tracing/debugging: without PIDE command context, messages are not shown, and Exn.trace hardly works anyway (see also de20fccf6509 and 447972249785);
prefer Isabelle_Thread.fork;
src/Pure
src/FOL
src/HOL
src/ZF
src/CCL
src/CTT
src/Cube
src/FOLP
src/LCF
src/Sequents
src/Doc
src/Tools