clarified execution graph traversal: stable imports are required to proceed, e.g. relevant to avoid crash of init_theory after discontinued execution;

tuned whitespace;

more toplevel pretty printing;

back to full synchronization (cf. eb3e399f5b9f);

proper sequential version;

eliminated Document.execution frontier (again, see 627fb639a2d9): just run into older execution, potentially stalling worker thread, but without global delay due to long-running tasks (notably sledgehammer); clarified Command.run_process etc.: join running eval, bypass running print; eliminated Command.memo in favour of regular Lazy.lazy; more Lazy.lazy status information;

modernized historic example;

more thorough Lazy.is_finished;