@SuppressWarnings("checkstyle:AvoidEscapedUnicodeCharacters")
    private boolean isForbidden(final char c) {
        // surrogates, U+FFFE and U+FFFF are forbidden in XML.
        return c == INVALID_CHARACTER_1 || c == INVALID_CHARACTER_2;
    }
Determines if the supplied characters is forbidden in XML, even as a character reference.

@param c The char to test.
@return True if the char is forbidden.
stroom-util/src/main/java/stroom/util/xml/MyXmlEmitter.java