Add explicit OR operator to sequence definition syntax

For example:

```
sequence
  a b c
```

means `a` followed by 'b' followed by 'c', but:

```
sequence
  (a b c)
```

means `a` or `b` or `c`. Better would probably be to add an explicit or operator such as `|` and use the parentheses only for grouping:

```
sequence
  a | b | c
```
