More than one type variable in a type variable list

Hi there :-) I'm a novice Lem user with a (possibly silly) question.

According to the grammar, it should be possible to specify a class with a type variable list.

I can get one type variable to work, ie

```
class (OS 'a)
   ...
end
```

but with 2 or more type variables

```
class (OS 'a 'b)
   ...
end
```
Lem complains `Syntax error`. Do you know what might be causing this?