code: public static final IEventFilter not(final IEventFilter filter)
      {
          return new AbstractEventFilter()
          {
              @Override
              public final boolean test(final GwtEvent<?> event)
              {
                  return (false == filter.test(event));
              }
          };
      }
docs: The resulting filter will return false, if the specified filter returns true.
  
  @param filter IEventFilter.
  @return IEventFilter