Simplify parser rules for left hand expressions

Hi Richard,

I propose here a small simplification of the parser. I've noticed that the rule:

```
   left_expr <- (
      <patt> / <ident>
   )
```

is not needed since <patt> include already the case of the simple identifier. I've therefore identified left_expr with patt and renamed this latter left_pattern to be more clear.

Francesco