report: existing data sets: add thoughts on the cause of holes

doc/report/applications.tex

@@ -51,8 +51,19 @@ remain unused in both Isabelle and Coq exports. A user formulating
 queries for ULO might be oblivious to the fact that only subsets of
 exports support given predicates.
 
-While not a problem for \emph{ulo-storage} per se, we do expect this
-to be a challenge when building a tetrapodal search
+We expect the difference between Coq and Isabelle exports to be caused
+by the difference in source material. It is only natural that
+different third party libraries expressed in different languages with
+different features will result in different ULO~predicates. However we
+want to just hint at the fact that this could also be an omission in
+the exporter code that originally generated the RDF~triplets we
+imported. This shows the importance of writing good importers,
+exporters take existing libraries and output ULO~triplets should lose
+as little information as possible to ensure good results for a larger
+tetrapodal search system.
+
+While these holes are not a problem for \emph{ulo-storage} per se, we
+do expect this to be a challenge when building a tetrapodal search
 system. Recommended ways around this ``missing fields'' problem in
 database literature include the clever use of default values or
 inference of missing values~\cite{kdisc, aidisc}, neither of which