public static boolean isXML11Valid(int c) {
        return (c < 0x10000 && (XML11CHARS[c] & MASK_XML11_VALID) != 0)
                || (0x10000 <= c && c <= 0x10FFFF);
    }
Returns true if the specified character is valid. This method
also checks the surrogate character range from 0x10000 to 0x10FFFF.
<p>
If the program chooses to apply the mask directly to the
<code>XML11CHARS</code> array, then they are responsible for checking
the surrogate character range.

@param c The character to check.
stroom-util/src/main/java/stroom/util/xml/XML11Char.java