new Function<AnEnum, AnEnum>() {
              boolean used = false;

              @Override
              public AnEnum apply(AnEnum ae) {
                checkState(!used, "should not be applied more than once");
                used = true;
                return ae;
              }
            });
    ImmutableMap<AnEnum, AnEnum> copy = Maps.immutableEnumMap(map);

            new Function<AnEnum, AnEnum>() {
              boolean used = false;

              @Override
              public AnEnum apply(AnEnum ae) {
                checkState(!used, "should not be applied more than once");
                used = true;
                return ae;
              }
            });
    ImmutableMap<AnEnum, AnEnum> copy = Maps.immutableEnumMap(map);