Dan,

In a document were you co-author:"A Query and Inference Service for RDF"  is said:
(In the introduction)

RDF provides a framework for representing machine-processable data on the Web. The RDF Model & Syntax specification [1] defines a formal set-theoretic data model which can have several isomorphic representations:

I am writing some access code to a RDF Object model (not so ambicious as to be said "RDF API), and cycles increase the complexity. On top of that, until now, I haven't found any example of RDF Data with a cycle inside. Of course I can make one, but it seems not to be used.

Thanks for your reaction,

Gerard.
 

Dan Brickley wrote:

Cycles are fine, even encouraged. The specialisation properties
(subClassOf, subPropertyOf) aren't supposed to loop, but that's all
the spec says w.r.t. restricting cycles. --dan