diff --git a/data-lens-fset.lisp b/data-lens-fset.lisp
index 7465e49..c953f37 100644
--- a/data-lens-fset.lisp
+++ b/data-lens-fset.lisp
@@ -17,9 +17,9 @@
   (data-lens.transducers:transducer-lambda
     ((acc next)
      (fset:with acc next))))
-(defmethod data-lens:functionalize ((it fset:set))
-  (lambda (key)
-    (nth-value 1 (fset:lookup it key))))
+(defmethod data-lens:functionalize ((set fset:set))
+  (lambda (it)
+    (fset:contains? set it)))
 (defmethod data-lens:extract-key ((it fset:set) key)
   (nth-value 1 (fset:lookup it key)))
 (defun make-set-lens (item)
@@ -63,6 +63,11 @@
            rec))
 
 
+(defmethod data-lens.transducers.internals:reduce-generic ((map fset:map) (func function) init)
+  (fset:reduce (lambda (acc k v)
+                 (funcall func acc (list k v)))
+               map
+               :initial-value init))
 (defmethod data-lens.transducers.internals:builder-for-input ((map fset:map))
   (values 'fset-map-builder
           map))
@@ -104,7 +109,9 @@
             m)))
 (defun make-bag-lens (item)
   (make-set-lens item))
+
 (defmethod data-lens.lenses:generic-lens ((rec fset:bag) cb loc)
   (funcall (funcall (make-set-lens loc)
                     cb)
            rec))
+
diff --git a/optics.lisp b/optics.lisp
index 8e9009b..92a8bcb 100644
--- a/optics.lisp
+++ b/optics.lisp
@@ -210,10 +210,26 @@ contain the new value at the location focused by the lens."
   (defun a-lens (cb)
     (lambda (foo)
       (fw.lu:prog1-bind (new (clone foo))
-                        (setf (a new)
-                              (funcall cb (a foo))))))
+        (setf (a new)
+              (funcall cb (a foo))))))
   (view 'a-lens
         (over 'a-lens '1+
               (set 'a-lens 2
                    (make-instance 'foo :a 1)))) #|
   ==> 3 |#)
+
+(defgeneric generic-lens (rec cb loc)
+  (:method ((rec hash-table) cb loc)
+    (funcall (funcall (make-hash-table-lens loc)
+                      cb)
+             rec))
+  (:method ((rec vector) cb loc)
+    (funcall (funcall (make-list-lens loc)
+                      cb)
+             rec)))
+
+(defun lens (loc)
+  "extensible lens using a multimethod for internal implementation"
+  (lambda (cb)
+    (lambda (rec)
+      (generic-lens rec cb loc))))