[x] upstream diff in mmt isabelle exporter

doc/timeline/week24.txt

 Week 24 (08.06. - 14.06.)
 =========================
 
-[ ] mmt: upstream changes to importers s.t. we don't get faulty data
+[x] mmt: upstream changes to importers s.t. we don't get faulty data
     in the future
 
+    - https://github.com/UniFormal/MMT/pull/521
+
 [x] supervision directory: clean up, find reasonable nomenclature
 
     - use ulo-storage-* as a prefix for all stuff related to making