zip = fn|a|
   map = a.map
   any = a.any
   next = |i| i()
   is_empty = |x| x==empty

   a = map(a;iter)
   return fn*||
      while true
         t = map(a;next)
         if any(t;is_empty)
            return empty
         else
            yield t
         end
      end
   end
end

Iterable.at = fn at|n|
   return self.skip(n)()
end