src/Pure/pure_thy.ML

     ("_index",      typ "logic => index",              Delimfix "(00\\<^bsub>_\\<^esub>)"),
     ("_indexdefault", typ "index",                     Delimfix ""),
     ("_indexvar",   typ "index",                       Delimfix "'\\<index>"),
     ("_struct",     typ "index => logic",              Mixfix ("\\<struct>_", [1000], 1000)),
     ("_update_name", typ "idt",                        NoSyn),
+    (Syntax.constrainAbsC, typ "'a",                   NoSyn),
     (const "==>",   typ "prop => prop => prop",        Delimfix "op ==>"),
     (const Term.dummy_patternN, typ "aprop",           Delimfix "'_"),
     ("_sort_constraint", typ "type => prop",           Delimfix "(1SORT'_CONSTRAINT/(1'(_')))"),
     (const "Pure.term", typ "logic => prop",           Delimfix "TERM _"),
     (const "Pure.conjunction", typ "prop => prop => prop", Infixr ("&&&", 2))]