wenzelm [Sat, 19 Jan 2013 22:17:26 +0100] rev 50987
always close derivation at the bottom of forked proofs (despite increased non-determinism of proof terms) -- improve parallel performance by avoiding dynamic dependency within large Isar proofs, e.g. Slicing/JinjaVM/SemanticsWF.thy in AFP/bf9b14cbc707;