docs: def getFirstUrn(self, reference=None):
          """ Get the first children URN for a given resource
  
          :param reference: CtsReference from which to find child (If None, find first reference)
          :type reference: CtsReference, str
          :return: Children URN
          :rtype: URN
          """
          if reference is not None:
              if ":" in reference:
                  urn = reference
              else:
                  urn = "{}:{}".format(
                      str(URN(str(self.urn)).upTo(URN.NO_PASSAGE)),
                      str(reference)
                  )
          else:
              urn = str(self.urn)
          _first = _SharedMethod.firstUrn(
              self.retriever.getFirstUrn(
                  urn
              )
          )
          return _first