The PortGroup and LinkGroup concepts require a "set of labels" concept.
E.g. "VLAN 8,42,100-119".

Here are two proposals how to represent this. Please pick one (personal
preference, XML/RDF-fetish, or throwing a dice are valid selection
mechanisms for today).

Option 2bis.
=========

XML:

  <nml:PortGroup id="urn:ogf:network:example.net:2010:mytrunk">
    <nml:labelgroup labeltype="http://schemas.ogf.org/nml/ethernet/2013/10/vlan">8,42,100-119</nml:labelgroup>
  </nml:PortGroup>

RDF:

  @prefix ex: <urn:ogf:network:example.net:2010> .
  @prefix nml: <http://example.ogf.org/schemas/nml/2013/03/base#>; .
  @prefix nmleth: <http://example.ogf.org/schemas/nml/2013/03/ethernet#>; .

  ex:mytrunk  a               nml:PortGroup .
  ex:mytrunk  nml:label       #12345678 .
  #12345678   nmlparam:type   "http://example.ogf.org/schemas/nml/2013/03/ethernet#vlan"  .
  #12345678   nmlparam:values "8,42,100-119" .

* The exact RDF syntax depends on the outcome of artf6495.

* The - and , are special characters in the label string.
* Technology-specific schemes must define the exact syntax of the label and the labelgroup string with parsing 
guidelines. (E.g. the WDM label string may contain the spacing)
* This proposal explicitly forbid negative labels to avoid parsing errors later.