when-lets.clj

(defmacro when-lets [bindings & body]
  (reduce (fn [acc tstform]
            `(when-let ~tstform ~acc))
          (cons 'do body)
          (reverse (map vec (partition 2 bindings)))))

(when-lets [n (first sizes)
	    s (seq coll)]
     (cons (take n coll)
	   (partitions (next sizes) (drop n coll)))))

(when-let [n (first sizes)]
          (when-let [s (seq coll)]
                    (do (cons (take n coll) (partitions (next sizes) (drop n coll))))))