diff --git a/src/wormhole/_allocator.py b/src/wormhole/_allocator.py
index 0644c55..399ced8 100644
--- a/src/wormhole/_allocator.py
+++ b/src/wormhole/_allocator.py
@@ -10,7 +10,7 @@ from . import _interfaces
 class Allocator(object):
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def wire(self, rendezvous_connector, code):
         self._RC = _interfaces.IRendezvousConnector(rendezvous_connector)
diff --git a/src/wormhole/_boss.py b/src/wormhole/_boss.py
index 2c93f0b..2a1d2db 100644
--- a/src/wormhole/_boss.py
+++ b/src/wormhole/_boss.py
@@ -39,7 +39,7 @@ class Boss(object):
     _tor_manager = attrib() # TODO: ITorManager or None
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._build_workers()
@@ -92,25 +92,25 @@ class Boss(object):
                  "RC": self._RC, "L": self._L, "C": self._C,
                  "T": self._T}
         for machine in which.split():
-            def tracer(old_state, input, new_state, output, machine=machine):
-                if output is None:
-                    if new_state:
-                        print("%s.%s[%s].%s -> [%s]" %
-                              (client_name, machine, old_state, input,
-                               new_state), file=file)
-                    else:
-                        # the RendezvousConnector emits message events as if
-                        # they were state transitions, except that old_state
-                        # and new_state are empty strings. "input" is one of
-                        # R.connected, R.rx(type phase+side), R.tx(type
-                        # phase), R.lost .
-                        print("%s.%s.%s" % (client_name, machine, input),
-                              file=file)
+            def tracer(old_state, input, new_state, machine=machine):
+                if new_state:
+                    print("%s.%s[%s].%s -> [%s]" %
+                          (client_name, machine, old_state, input,
+                           new_state), file=file)
                 else:
-                    if new_state:
-                        print(" %s.%s.%s()" % (client_name, machine, output),
-                              file=file)
+                    # the RendezvousConnector emits message events as if
+                    # they were state transitions, except that old_state
+                    # and new_state are empty strings. "input" is one of
+                    # R.connected, R.rx(type phase+side), R.tx(type
+                    # phase), R.lost .
+                    print("%s.%s.%s" % (client_name, machine, input),
+                          file=file)
                 file.flush()
+                def output_tracer(output):
+                    print(" %s.%s.%s()" % (client_name, machine, output),
+                          file=file)
+                    file.flush()
+                return output_tracer
             names[machine].set_trace(tracer)
 
     ## def serialize(self):
diff --git a/src/wormhole/_code.py b/src/wormhole/_code.py
index b2a9a20..1eeaa1a 100644
--- a/src/wormhole/_code.py
+++ b/src/wormhole/_code.py
@@ -13,7 +13,7 @@ def first(outputs):
 class Code(object):
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def wire(self, boss, allocator, nameplate, key, input):
         self._B = _interfaces.IBoss(boss)
diff --git a/src/wormhole/_input.py b/src/wormhole/_input.py
index 6985253..7fe8afd 100644
--- a/src/wormhole/_input.py
+++ b/src/wormhole/_input.py
@@ -14,7 +14,7 @@ def first(outputs):
 class Input(object):
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._all_nameplates = set()
diff --git a/src/wormhole/_key.py b/src/wormhole/_key.py
index 91c972f..368090a 100644
--- a/src/wormhole/_key.py
+++ b/src/wormhole/_key.py
@@ -64,7 +64,7 @@ class Key(object):
     _side = attrib(validator=instance_of(type(u"")))
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._SK = _SortedKey(self._appid, self._versions, self._side,
@@ -115,7 +115,7 @@ class _SortedKey(object):
     _side = attrib(validator=instance_of(type(u"")))
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def wire(self, boss, mailbox, receive):
         self._B = _interfaces.IBoss(boss)
diff --git a/src/wormhole/_lister.py b/src/wormhole/_lister.py
index cd1a560..0478046 100644
--- a/src/wormhole/_lister.py
+++ b/src/wormhole/_lister.py
@@ -10,7 +10,7 @@ from . import _interfaces
 class Lister(object):
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def wire(self, rendezvous_connector, input):
         self._RC = _interfaces.IRendezvousConnector(rendezvous_connector)
diff --git a/src/wormhole/_mailbox.py b/src/wormhole/_mailbox.py
index 3bca6fb..db52d36 100644
--- a/src/wormhole/_mailbox.py
+++ b/src/wormhole/_mailbox.py
@@ -10,7 +10,7 @@ from . import _interfaces
 class Mailbox(object):
     _side = attrib(validator=instance_of(type(u"")))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._mailbox = None
diff --git a/src/wormhole/_nameplate.py b/src/wormhole/_nameplate.py
index 8ee8025..a8f9278 100644
--- a/src/wormhole/_nameplate.py
+++ b/src/wormhole/_nameplate.py
@@ -7,7 +7,7 @@ from ._wordlist import PGPWordList
 @implementer(_interfaces.INameplate)
 class Nameplate(object):
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __init__(self):
         self._nameplate = None
diff --git a/src/wormhole/_order.py b/src/wormhole/_order.py
index 5383a14..31377cb 100644
--- a/src/wormhole/_order.py
+++ b/src/wormhole/_order.py
@@ -11,7 +11,7 @@ class Order(object):
     _side = attrib(validator=instance_of(type(u"")))
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._key = None
diff --git a/src/wormhole/_receive.py b/src/wormhole/_receive.py
index 7003110..8cb88c2 100644
--- a/src/wormhole/_receive.py
+++ b/src/wormhole/_receive.py
@@ -12,7 +12,7 @@ class Receive(object):
     _side = attrib(validator=instance_of(type(u"")))
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._key = None
diff --git a/src/wormhole/_rendezvous.py b/src/wormhole/_rendezvous.py
index ec30b45..bb1ece4 100644
--- a/src/wormhole/_rendezvous.py
+++ b/src/wormhole/_rendezvous.py
@@ -83,7 +83,7 @@ class RendezvousConnector(object):
         self._trace = f
     def _debug(self, what):
         if self._trace:
-            self._trace(old_state="", input=what, new_state="", output=None)
+            self._trace(old_state="", input=what, new_state="")
 
     def _make_endpoint(self, hostname, port):
         if self._tor_manager:
diff --git a/src/wormhole/_send.py b/src/wormhole/_send.py
index 762b2fa..b039dbd 100644
--- a/src/wormhole/_send.py
+++ b/src/wormhole/_send.py
@@ -12,7 +12,7 @@ class Send(object):
     _side = attrib(validator=instance_of(type(u"")))
     _timing = attrib(validator=provides(_interfaces.ITiming))
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __attrs_post_init__(self):
         self._queue = []
diff --git a/src/wormhole/_terminator.py b/src/wormhole/_terminator.py
index f90f7e5..e6c65a0 100644
--- a/src/wormhole/_terminator.py
+++ b/src/wormhole/_terminator.py
@@ -6,7 +6,7 @@ from . import _interfaces
 @implementer(_interfaces.ITerminator)
 class Terminator(object):
     m = MethodicalMachine()
-    set_trace = getattr(m, "setTrace", lambda self, f: None)
+    set_trace = getattr(m, "_setTrace", lambda self, f: None)
 
     def __init__(self):
         self._mood = None