Determinism

We need to check for determinism, meaning that the matches must not have intersecting pattern.

```
let rec choice(n) = match n with 
  | Nil -> Eps  
  | Cons(a,m) -> match m with 
    | Nil -> a 
    | Cons(b,bs) -> choice(m)
    | Cons(b,bs) -> a  (* non determinism*)
```

this should be rejected