public static boolean isPrefix(String in){

        if(in.length() == 1)
            for(int i = 0; i < vars.length; i++)
                if(in.equals(vars[i]))
                    return true;
        else {
                if(in.startsWith("/") || in.startsWith("*") ||
                   in.startsWith("+") || in.startsWith("-")){

                for(int k = 1; k < in.length(); k++){
                    if(isPrefix(in.substring(1,k)) &&
                       isPrefix(in.substring(k,in.length())))
                            return true;
                }
                }
        }
        return false;
    }