}
        return keys;
      }
    }

    @Override
    // A forwarding implementation can't do any better than the underlying object.
    @SuppressWarnings("UndefinedEquals")
    public boolean equals(@Nullable Object o) {
      if (o == this) {
        return true;
      }
      synchronized (mutex) {
        return delegate().equals(o);
      }
    }

        }
        return keys;
      }
    }

    @Override
    // A forwarding implementation can't do any better than the underlying object.
    @SuppressWarnings("UndefinedEquals")
    public boolean equals(@Nullable Object o) {
      if (o == this) {
        return true;
      }
      synchronized (mutex) {
        return delegate().equals(o);
      }
    }