wenzelm [Sat, 04 Nov 2006 19:25:43 +0100] rev 21177
removed checkpoint interface;
moved back to copy/checkpoint instead of checkpoint/checkpoint
(NB 1: checkpoint is idempotent, i.e. impure data is being shared, which is inappropriate;
NB 2: copying a checkpoint always produces a related theory);
present_proof: proper treatment of history;
tuned;