2011-06-30 ago more general theory header parsing;
2011-06-18 ago more uniform treatment of "keyword" vs. "operator";
2011-06-17 ago recovered markup for non-alphabetic keywords;
2010-10-30 ago support for floating-point tokens in outer syntax (coinciding with inner syntax version);
2010-08-12 ago more basic notion of unparsed input;
2010-05-17 ago renamed class Outer_Lex to Token and Token_Kind to Token.Kind;