So far we have identified several different relations. We have defined them all in a single direction (hasTopology, 
hasNode, hasPort, isSource, isSink, etc.).

The question now is whether we want to define the inverse of relations as well.

E.g. the inverse of isSource is hasSource:

<nml:Link id="urn:ogf:network:provider.com:2012:PortA">
  <nml:Relation type="isSource">
    <nml:Port idRef="urn:ogf:network:client.net:2012:LinkA"/>
  </nml:Relation>
</nml:Link>

<nml:Port id="urn:ogf:network:client.net:2012:LinkA"/>
  <nml:Relation type="hasSource">
    <nml:Link idRef="urn:ogf:network:provider.com:2012:PortA">
  </nml:Relation>
</nml:Link>