2010-11-13 ago somewhat adhoc replacement for 'thus' and 'hence';
2010-11-10 ago eliminated obsolete heading category -- superseded by heading_level;
2010-11-10 ago treat main theory commands like headings, and nest anything else inside;
2010-11-10 ago default Sidekick parser based on section headings;
2010-11-10 ago some support for nested source structure, based on section headings;
2010-08-17 ago report command token name instead of kind, which can be retrieved later via Outer_Syntax.keyword_kind;
2010-05-17 ago renamed class Outer_Lex to Token and Token_Kind to Token.Kind;
2010-05-15 ago renamed Outer_Keyword to Keyword (in Scala);
2010-01-05 ago tuned message;
2009-12-22 ago renamed class Outer_Keyword to Outer_Syntax;