function nextInstanceId() {
        var LETTERS_CNT = (90 /* Z */ - 65 /* A */ + 1);
        var result = _INSTANCE_COUNT++;
        result = result % (2 * LETTERS_CNT);
        if (result < LETTERS_CNT) {
            return String.fromCharCode(97 /* a */ + result);
        }
        return String.fromCharCode(65 /* A */ + result - LETTERS_CNT);
    }
Produces 'a'-'z', followed by 'A'-'Z'... followed by 'a'-'z', etc.