diff --git a/csys.lisp b/csys.lisp
index 2cfaab9..4762994 100644
--- a/csys.lisp
+++ b/csys.lisp
@@ -5,7 +5,8 @@
   (:local-nicknames (:actor :scopes/core/actor)
                     (:util :scopes/util))
   (:export #:neuron #:synapse
-           #:forward))
+           #:forward
+           #:set-content))
 
 (in-package :decons/csys)
 
@@ -22,3 +23,10 @@
 (defun forward (msg state syns env)
   (dolist (s syns)
     (funcall s msg)))
+
+;;;; helpers for operating on messages
+
+(defun set-content (fn)
+  (lambda (msg) (actor:message 
+                  (funcall fn (actor:content msg)) 
+                  (actor:customer msg))))
diff --git a/test-csys.lisp b/test-csys.lisp
index 814229c..a339d25 100644
--- a/test-csys.lisp
+++ b/test-csys.lisp
@@ -9,10 +9,11 @@
   ))
 
 (deftest test-neuron ()
-  (let* ((prb (csys:neuron #'probe :env t:*test-suite* :state '(42 43)))
-         (fw (csys:neuron #'csys:forward :syns (list (csys:synapse prb)))))
-    (actor:send fw (actor:message 42))
-    (actor:send fw (actor:message 43))
+  (let* ((prb (csys:neuron #'probe :env t:*test-suite* :state '(43 44)))
+         (syn (csys:synapse prb (csys:set-content (lambda (c) (+ 1 c)))))
+         (fw (csys:neuron #'csys:forward :syns (list syn))))
+    (actor:send fw 42)
+    (actor:send fw 43)
     (sleep 0.1)
     (actor:stop prb)
     (actor:stop fw)