static Class<?> getElementTypeFromEnumSet(Object enumSet)
    {
        if (__elementTypeFromEnumSet == null)
        {
            throw new RuntimeException(
                    "Could not access (reflection) the private "
                            + "field *elementType* (enumClass) from: class java.util.EnumSet");
        }

        try
        {
            return (Class<?>) __elementTypeFromEnumSet.get(enumSet);
        }
        catch (IllegalArgumentException e)
        {
            throw new RuntimeException(e);
        }
        catch (IllegalAccessException e)
        {
            throw new RuntimeException(e);
        }
    }
Retrieves the enum key type from the EnumMap via reflection. This is used by {@link ObjectSchema}.
protostuff-runtime/src/main/java/io/protostuff/runtime/EnumIO.java