ctyp: added 'sorts' field;
may_insert_typ/term/env_sorts: observe Sign.all_sorts_nonempty;
may_insert_env_sorts: insert sorts of type subst only;
instantiate: insert sorts of insts;
tuned;
* May 2005: Rafal Kolanski, NICTA
Substantially improved retrieval of facts from theory/proof context.
* May 2005: Florian Haftmann, TUM
Several new antiquotation.
* February 2005: Lucas Dixon, University of Edinburgh
Substantially improved subst method.
$Id$