I changed:
    nml:hasInboundPort a nml:Relation .
to
    nml:hasInboundPort rdfs:subPropertyOf nml:Relation .

After discussion with Jeroen.

fine with me

RDF does not differentiate between NML relations and other relations.

A) If a relation is translated from RDF to XML, how to determine if the relation is a NML relation or not?

E.g. the following code 

 @prefix nml: <http://schemas.ogf.org/nml/2013/10/base#>;
 @prefix nmlrel: <http://schemas.ogf.org/nml/2013/10/relation#>;
 urn:ogf:network:example.net:2012:mynode a nml:Node
 urn:ogf:network:example.net:2012:mynode nmlrel:hasInboundPort urn:ogf:network:example.net:2012:myport
 urn:ogf:network:example.net:2012:myport a nml:Port

Can either translate to:

 <nml:Node id="urn:ogf:network:example.net:2012:mynode">
   <nmlrel:hasInboundPort>
     <nml:Port idRef="urn:ogf:network:example.net:2012:myport" />
   </nmlrel:hasInboundPort>
 </nml:Node>

or to:

 <nml:Node id="urn:ogf:network:example.net:2012:mynode">
   <nml:Relation type="http://schemas.ogf.org/nml/2013/10/relation#hasInboundPort">;
     <nml:Port idRef="urn:ogf:network:example.net:2012:myport" />
   </nml:Relation>
 </nml:Node>

Options:

1. Use nml:Relation for relations in the http://schemas.ogf.org/nml namespace.
2. Use nml:Relation for a set of predefined relations.
3. Add some syntax to RDF to differentiate between a nml:Relation and some other relation
4. Never use a nml:Relation, even not for relations in the http://schemas.ogf.org/nml namespace.
5. Always use a nml:Relation, even for relations outside the http://schemas.ogf.org/nml namespace.

RDF regards an NML relations as any other property. In order to distinguish in RDF between 

    <nml:Relation type="http://schemas.ogf.org/nml/2013/10/base#hasInboundPort">;
and
    <nml:hasInboundPort>

Propose: RDF needs to add the following triplet for all NML relations:
    nml:hasInboundPort a nml:Relation .

Note: a NML version 1 compliant client does not need to add the these triplets for relations defined in version 1 of NML
, but it MUST include them for other relations if they are used in the description. Eg. if it is using a relation 
defined in the experimental extension schema.


Example 1: the following two syntaxes are equivalent.

RDF:

 @prefix nml: <http://schemas.ogf.org/nml/2013/10/base#>; .
 nml:hasInboundPort a nml:Relation .
 urn:ogf:network:example.net:2012:mynode a nml:Node .
 urn:ogf:network:example.net:2012:mynode nml:hasInboundPort urn:ogf:network:example.net:2012:myport .
 urn:ogf:network:example.net:2012:myport a nml:Port .

XML:

 <nml:Node id="urn:ogf:network:example.net:2012:mynode">
   <nml:Relation type="http://schemas.ogf.org/nml/2013/10/base#hasInboundPort">;
     <nml:Port idRef="urn:ogf:network:example.net:2012:myport" />
   </nml:Relation>
 </nml:Node>

Example 2: the following two syntaxes are equivalent (note the missing "nml:hasInboundPort a nml:Relation ." line)

RDF:

 @prefix nml: <http://schemas.ogf.org/nml/2013/10/base#>; .
 urn:ogf:network:example.net:2012:mynode a nml:Node .
 urn:ogf:network:example.net:2012:mynode nml:hasInboundPort urn:ogf:network:example.net:2012:myport .
 urn:ogf:network:example.net:2012:myport a nml:Port .

XML:

 <nml:Node id="urn:ogf:network:example.net:2012:mynode">
   <nml:hasInboundPort">
     <nml:Port idRef="urn:ogf:network:example.net:2012:myport" />
   </nml:hasInboundPort>
 </nml:Node>

I propose the following solution:

* A NML version x compliant renderer MUST use the second NML Relation syntax for all relations defined in NML version x. (Thus the parser MUST have a 
list of known NML Relations)
* A RDF description MAY contain an explicit line to signify that something is a NML relation. Either:
  nmlrel:hasInboundPort rdfs:subPropertyOf nml:Relation
* A RDF parser/XML renderer which detects the above line SHOULD use the second NML Relation syntax for this relation.
* In all other cases, the XML renderer MUST use the first relation syntax (thus not use <nml:Relation>)

A question for the RDF folks. What is better:
  nmlrel:hasInboundPort rdfs:subPropertyOf nml:Relation
or
  nmlrel:hasInboundPort a nml:Relation