code: private void validateConfiguration(ArquillianDescriptor desc) {
          Object defaultConfig = null;
  
          // verify only one container is marked as default
          for (ContainerDef container : desc.getContainers()) {
              if (container.isDefault()) {
                  if (defaultConfig != null) {
                      throw new IllegalStateException("Multiple Containers defined as default, only one is allowed:\n"
                          + defaultConfig
                          + ":"
                          + container);
                  }
                  defaultConfig = container;
              }
          }
          boolean containerMarkedAsDefault = defaultConfig != null;
  
          // verify only one container or group is marked as default
          for (GroupDef group : desc.getGroups()) {
              if (group.isGroupDefault()) {
                  if (defaultConfig != null) {
                      if (containerMarkedAsDefault) {
                          throw new IllegalStateException(
                              "Multiple Containers/Groups defined as default, only one is allowed:\n"
                                  + defaultConfig
                                  + ":"
                                  + group);
                      }
                      throw new IllegalStateException(
                          "Multiple Groups defined as default, only one is allowed:\n" + defaultConfig + ":" + group);
                  }
                  defaultConfig = group;
              }
  
              ContainerDef defaultInGroup = null;
              // verify only one container in group is marked as default
              for (ContainerDef container : group.getGroupContainers()) {
                  if (container.isDefault()) {
                      if (defaultInGroup != null) {
                          throw new IllegalStateException(
                              "Multiple Containers within Group defined as default, only one is allowed:\n" + group);
                      }
                      defaultInGroup = container;
                  }
              }
          }
      }
docs: Validate that the Configuration given is sane
  
  @param desc
  The read Descriptor