When one topology is contained within the other, the default implicit relation is hasTopology:

 <nml:Topology id="urn:ogf:network:nordu.net:2012:NORDUnet">
   <nml:Topology id="urn:ogf:network:nordu.net:2012:NORDUnet:Scandinavia"/>
 </nml:Topology>

is equivalent to

 <nml:Topology id="urn:ogf:network:nordu.net:2012:NORDUnet">
   <nml:Relation type="http://schemas.ogf.org/nml/2012/10/relations#hasTopology">
     <nml:Topology id="urn:ogf:network:nordu.net:2012:NORDUnet:Scandinavia"/>
   </nml:Relation>
 </nml:Topology>

The following rules apply to implicit relations:
* A client SHOULD leave out the implicit relation when writing NML in XML format, but instead use a direct child element
.
* parsers MUST accept the implicit relation
* parsers SHOULD accept the explicit relation

See artf6547 for a full list of implicit relations.

See: https://forge.ogf.org/integration/viewcvs/viewcvs.cgi/*checkout*/201203-subtopology/hasTopology.xml?rev=20&content-

type=text%2Fplain&system=exsy1001&root=nml-examples