src/Pure/Isar/keyword.scala
2010-11-10 ago eliminated obsolete heading category -- superseded by heading_level;
2010-11-10 ago added missing Keyword.THY_SCHEMATIC_GOAL;
2010-08-08 ago explicitly distinguish Output.status (essential feedback) vs. Output.report (useful markup);
2010-08-07 ago simplified type XML.Tree: embed Markup directly, avoid slightly odd triple;
2010-05-15 ago renamed Outer_Keyword to Keyword (in Scala);