Currently, an NML Relation is defined as a relation between Network Objects.

This is a problem for relations between non-Network Objects, like the "locatedAt" relation which related a Network 
Object to a Location. However, Location is not a subclass of Network Object. Hence, this relation is formally invalid. 
Another example is in RDF, where the "next" relation is used between List Items, which are not Network Objects.

We can solve this three ways:
1) Make every a subclass of a Network Object
2) Remove the restriction that a NML Relation must be between Network Objects.
3) Make a distinction between NML Relation and other type of Relations.

For simplicity sake, I propose solution (2). A NML Relation (if we still like to define such beast) would be "any 
relation that is defined by the NML specs" rather than "any relation between two NML Network Objects".