docs: @Exported
      public String getDescription() {
          Set<Node> nodes = getNodes();
          if(nodes.isEmpty()) {
              Set<Cloud> clouds = getClouds();
              if(clouds.isEmpty())
                  return Messages.Label_InvalidLabel();
  
              return Messages.Label_ProvisionedFrom(toString(clouds));
          }
  
          if(nodes.size()==1)
              return nodes.iterator().next().getNodeDescription();
  
          return Messages.Label_GroupOf(toString(nodes));
      }