[Vala] how to pass a List by reference?

[bsquared <bwcode4u gmail com>]

I am guessing that I'm going about this wrong, but I want to populate a list
from a static method.

     static void populate_list (ref SList<string> list) {
            list.append("");
     }

I am getting a compiler error:
error: Cannot capture reference or output parameter

I want the ownership of the list to belong to the calling method.

What am I doing wrong?

-- 
Regards,
Brian Winfrey