/**
 * Appends a value to an array if the value does not already exist within the array.
 * 
 * @param a1      The array to modify. 
 * @param val      The value to append. 
 * @return Returns a modified array or an error string. 
 * @author Craig Fisher (craig@altainteractive.com) 
 * @version 1, October 29, 2001 
 */
 function ArrayAppendUnique(a1,val) {
    if ((NOT IsArray(a1))) {
        writeoutput("Error in <Code>ArrayAppendUnique()</code>! Correct usage: ArrayAppendUnique(<I>Array</I>, <I>Value</I>) -- Appends <em>Value</em> to the array if <em>Value</em> does not already exist");
        return 0;
    }
    if (NOT ListFind(Arraytolist(a1), val)) {
        arrayAppend(a1, val);
    }
    return a1;
}