Fix unenqueue

Don't lose the penultimate element when dequeuing one by one.

collectors.lisp

@@ -366,18 +366,18 @@ FUNCTION and INITIAL-VALUE are passed directly to MAKE-REDUCER."
          (len (length head))
          (div (- len (+ 1 n)))
          (rtn (cond
-                ((plusp div)
+                ((minusp div)
+                 (setf (value o) nil)
+                 (when (typep o 'collector)
+                   (setf (tail o) nil))
+                 head)
+                (t
                  (let* ((c (nthcdr div head))
                         (rtn (cdr c)))
                    (setf (cdr c) nil)
                    (when (typep o 'collector)
                      (setf (tail o) c))
-                   rtn))
-                (t
-                 (setf (value o) nil)
-                 (when (typep o 'collector)
-                   (setf (tail o) nil))
-                 head))))
+                   rtn)))))
     (if (= 1 n)
         (car rtn)
         rtn)))

tests/collectors.lisp

@@ -252,5 +252,9 @@
     (it 1 2 3 4 5 6)
     (assert-equal 6 (unenqueue-it))
     (assert-equal '(4 5) (unenqueue-it 2))
-    (assert-equal '(1 2 3) (it))
-    ))
+    (assert-equal '(1 2 3) (it)))
+  (with-collector (it)
+    (it 0 1 2)
+    (assert-equal 2 (unenqueue-it))
+    (assert-equal 1 (unenqueue-it))
+    (assert-equal 0 (unenqueue-it))))