public static int supplemental(char h, char l) {
        return (h - 0xD800) * 0x400 + (l - 0xDC00) + 0x10000;
    }
Returns true the supplemental character corresponding to the given
surrogates.

@param h The high surrogate.
@param l The low surrogate.
stroom-util/src/main/java/stroom/util/xml/XMLChar.java