src/Pure/Isar/proof_history.ML
2000-05-05 ago GPLed;
1999-10-05 ago clear: int argument;
1999-08-26 ago back: recur flag;
1999-07-06 ago removed nesting (unused);
1999-05-21 ago adapted to History changes;
1999-03-17 ago added apply_cond_open;
1998-11-21 ago added undos, redos;
1998-11-09 ago Histories of proof states, with undo / redo and prev / back.