src/Pure/Thy/thy_read.ML

       val dummy = unlink_thy child;
       val mergelist = load_base bases;
 
       val base_thy =
        (writeln ("Loading theory " ^ quote child);
-        Theory.merge_list (map theory_of mergelist));
+        Theory.prep_ext_merge (map theory_of mergelist));
 
       val datas =
         let fun get_data t =
               let val ThyInfo {data, ...} = the (get_thyinfo t)
               in snd data end;