code: def node_pairing(self):
          
          value = self.attributes['node_pairing']
          if value not in IMB.NODE_PAIRING:
              msg = 'Unexpected {0} value: got "{1}" but valid values are {2}'
              msg = msg.format('node_pairing', value, IMB.NODE_PAIRING)
              raise ValueError(msg)
          return value
docs: if "node" then test current node and next one
          if "tag", then create tests for every pair of the current tag.