+ example to reproduce issue 24
* fixing the incorrect behavior of transitive reduction when Top only
occurs positively and thus isn't derived for all contexts (but is
derived for some, for example, due to tautological axioms). Now the
reduction explicitly accounts for Top that it's always computed as a
direct subsumer in case there're no other subsumers
* improved error reporting for various incremental tests so that the
error message contains both the expected and the actual taxonomies
minor comment
creating default output for transitive reduction for owl:Thing to be
used when there are no (direct) subsumers found