*/
      private static final ClassValue<Boolean> isValidClass =
          new ClassValue<Boolean>() {
            @Override
            protected Boolean computeValue(Class<?> type) {
              checkExceptionClassValidity(type.asSubclass(Exception.class));
              return true;
            }
          };

      @Override
      public void validateClass(Class<? extends Exception> exceptionClass) {

import static com.google.common.util.concurrent.Futures.immediateFailedFuture;
import static com.google.common.util.concurrent.Futures.immediateFuture;
import static com.google.common.util.concurrent.FuturesGetChecked.checkExceptionClassValidity;
import static com.google.common.util.concurrent.FuturesGetChecked.getChecked;
import static com.google.common.util.concurrent.FuturesGetChecked.isCheckedException;

          if (exceptionClass.equals(knownGood.get())) {
            return;
          }
          // TODO(cpovirk): if reference has been cleared, remove it?
        }
        checkExceptionClassValidity(exceptionClass);

        /*
         * It's very unlikely that any loaded Futures class will see getChecked called with more