I don't suppose ```type := limited(<collection>, of: type)``` would always not blow up here?  :-)

I think (but am not certain) that this should be ```type := <deque>``` and it should attempt to do ```as(<deque>, default)``` so that the default can be supplied as a list or vector without blowing up.  The code in repeated options calls <deque>-specific methods so I think this code needs to match that.