optional labeled non-terminal leads to parser generator bug

```
syntax A = B label?
syntax B = "b"
```

produces:

```
rascal>[A] "b"
|prompt:///|(4,3,<1,4>,<1,7>): Java compilation failed due to method B() is already defined in class org.rascalmpl.java.parser.object.$root$_1884401898 at 228, 44 
```

while this just works:

```
syntax A = B? label
syntax B = "b"
````

