src/HOL/Tools/Sledgehammer/async_manager_legacy.ML
2016-04-02 ago prefer infix operations;
2015-11-03 ago clarified modules;
2015-10-02 ago further reduced dependency on legacy async thread manager
2015-07-21 ago more explicit thread identification;
2015-01-29 ago more explicit indication of Async_Manager_Legacy as Proof General legacy;