"Facts". […] A fact consists of an attribute: value pair. A value may be stated directly or referenced indirectly by means of another attribute having that value. Chains of reference may be arbitrarily long, though a single attribute may appear only once in any chain. If, at the end of a chain of reference, no value can be found for an attribute, the fact in which that attribute appears is undefined.
- Corbett & Fraser (2004), a pag.368