eliminated somewhat pointless parallelism (from 857da80611ab): usually hundreds of tasks with < 1ms each, also note that the enclosing join_theory happens within theory graph parallelism;
Notes on building Poly/ML as Isabelle component
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* component skeleton:
$ isabelle build_polyml_component -s sha1 component
* include full source (without symlink), for example:
$ wget https://github.com/polyml/polyml/archive/master.zip