removed junk (amending 8cd399b25dac);

more robust wrt. undefined state;

more informative error;

more robust;

tuned signature;

clarified signature: more explicit types;

more informative shasum: show differences explicitly;

tuned messages;

more robust ancestor_results: avoid total existence failure after build_process has crashed elsewhere;

clarified synchronized regions: avoid deadlock of Build_Cluster operations on other thread vs. return_code(), notably via capture() error handling;