Allow definition of subsequences to reduce duplication

Currently, the sequence is always one "sentence":

```
sequence
  (a b c)*
  d
  (a b c e)*
```

Which may lead, as shown above, to duplication (in this case, of `a b c`). A possible solution is to allow the definition of additional "production rules". As an example, this is illustrated in the following example:

```
def x = a b c
sequence = x* d (x e)*
```
