*** MESSAGE REFERS TO PREVIOUS VERSION *** support embedded values and static binding -- via implicit assignment to src tokens (cf. assignable/assign/closure); renamed ident/string/keyword to mk_ident/mk_string/mk_keyword; added mk_name, mk_typ, mk_term, mk_fact, mk_attribute; added type value with map_values etc.; removed name_dummy, added general 'maybe' combinator; added global/local_tyname/const; added pretty_src, pretty_attribs; added thm_sel (from attrib.ML);