The stack is not always emptied when the stream is consumed

If we don't have a "global mod" on the dice (like simply `3d6` instead of `3d6`) then the stack will still have at least `` on it when we finish.

For now we hard code checks for `` and ``, but we should actually run the parser to make sure they are compatible with an empty stream (as both of the above are). That way we won't have to maintain a list of hard-coded acceptable tokens.