Acyclic RDF Graph???

Hello!,

After working with RDF for some time, I find that it is not very clear
if cycles in the RDF graph are allowed or not. The RDF Data spec. does
not say anything about it (or I don't understand it as such), and the
restrictions in the RDF Schema spec, about of subPropertyof and
subClassof do not really constrain the presence of a loop in the a RDF
description.

Could somebody bring some light to my cloudy head??

Thanks,

Gerard.