public static int parseNonNegativeInteger(String attrVal) {
        int rv = parseInteger(attrVal);
        if (rv < 0) {
            return -1;
        } else {
            return rv;
        }
    }
Returns the non-negative integer represented by 
<code>attrVal</code> or -1 on error.

@param attrVal a string representing a non-negative 
integer attribute value (can be <code>null</code>)
@return the integer represented by <code>attrVal</code> or 
-1 on error
src/nu/validator/checker/AttributeUtil.java