Fixed many bugs.

test/book/test_PersistentSortedList.clj

@@ -30,6 +30,9 @@
   (fact "drop"
         (drop 2 (sorted-list 1 2 3)) => (sorted-list 3)
         (drop 3 (sorted-list 1 2 3)) => (list))
+  (fact "empty"
+        (empty (sorted-list 2 3 4)) => (sorted-list)
+        (into (empty (sorted-list-by #(- %2 %1) 2 3 4)) [20 30]) => (sorted-list-by #(- %2 %1) 20 30))
   (fact "reduce"
         (reduce + (sorted-list 1 2 3)) => 6
         (reduce + 10 (sorted-list 1 2 3)) => 16)