[Req] Please add a method to check grammar completeness

I often forget to define some rules.

something like

```
rule1 := rule2, rule3;
rule2 := "A";
```

rule3 is undefined in grammar, but I don't see that.
So the program should have a method which will check the grammar for completeness (or duplication of definitions), which I can call after grammar construction but before parsing.
