From [Ingo](http://iwmuser.uni-koblenz.de/~dahn/TLA/Experience/V1p0/xAPI-Spec_ID.pdf):

> The choice of display and omission of other possible properties of verbs is not clear.
For example Activity Stream verbs don’t have prescribed human readable forms. On the other hand, allowing semantic relations between verbs, like “is special case of” would allow for much more refined search for statements.