src/HOL/Isar_Examples/Group_Context.thy
2014-02-21 wenzelm 2014-02-21 more symbols;
2012-05-06 wenzelm 2012-05-06 tuned proofs;
2012-04-03 wenzelm 2012-04-03 close context elements via Expression.cert/read_declaration; ensure visible context;
2012-04-03 wenzelm 2012-04-03 some context examples;