Integer arg1 = trackInteger(expr.getArg(0));
                Integer arg2 = trackInteger(expr.getArg(1));
                UnaryOperation op;
                if (arg1 != null) {
                    if (arg2 != null) {
                        op = new Replace1((char) arg1.intValue(), (char) arg2.intValue());
                    } else {
                        op = new Replace2((char) arg1.intValue());
                    }
                } else {
                    if (arg2 != null) {