Add ordered set operations?

[mars0i]

Since, as noted in the ordered-set docstring, clojure.set/union and other operations in clojure.set may reorder elements for efficiency, perhaps it would be useful for ordered to include (less efficient?) set operations that preserve order. For example, I needed to preserve order with union, so I am using this definition (which might also be called "union"):

(defn multi-conj
  "Successively conj each element of ys onto xs."
  [xs ys]
  (reduce (fn [newxs y] (conj newxs y))
          xs ys))

(I need it for two-element sets on the right, so efficiency shouldn't be an issue in my case.)