diff --git a/doc/source/reference.rst b/doc/source/reference.rst
index d85b838..0f4fc94 100644
--- a/doc/source/reference.rst
+++ b/doc/source/reference.rst
@@ -4,6 +4,8 @@ Reference
 Module Base Classes
 ...................
 
+.. autodata:: secop.modules.Done
+
 .. autoclass:: secop.modules.Module
     :members: earlyInit, initModule, startModule, pollerClass
 
diff --git a/secop/lib/__init__.py b/secop/lib/__init__.py
index c0c21c7..e942ff5 100644
--- a/secop/lib/__init__.py
+++ b/secop/lib/__init__.py
@@ -31,9 +31,6 @@ from configparser import ConfigParser
 from os import environ, path
 
 
-unset_value = object()
-
-
 class GeneralConfig:
     def __init__(self):
         self._config = None
@@ -319,3 +316,11 @@ def formatStatusBits(sword, labels, start=0):
         if sword & (1 << i) and lbl:
             result.append(lbl)
     return result
+
+
+class UniqueObject:
+    def __init__(self, name):
+        self.name = name
+
+    def __repr__(self):
+        return 'UniqueObject(%r)' % self.name
diff --git a/secop/modules.py b/secop/modules.py
index 1cdca29..4793450 100644
--- a/secop/modules.py
+++ b/secop/modules.py
@@ -31,14 +31,17 @@ from secop.datatypes import ArrayOf, BoolType, EnumType, FloatRange, \
     IntRange, StatusType, StringType, TextType, TupleOf
 from secop.errors import BadValueError, ConfigError, \
     ProgrammingError, SECoPError, SilentError, secop_error
-from secop.lib import formatException, mkthread
+from secop.lib import formatException, mkthread, UniqueObject
 from secop.lib.enum import Enum
 from secop.params import Accessible, Command, Parameter
 from secop.poller import BasicPoller, Poller
 from secop.properties import HasProperties, Property
 from secop.logging import RemoteLogHandler, HasComlog
 
-Done = object()  #: a special return value for a read/write function indicating that the setter is triggered already
+Done = UniqueObject('already set')
+"""a special return value for a read/write function
+
+indicating that the setter is triggered already"""
 
 
 class HasAccessibles(HasProperties):
diff --git a/secop/params.py b/secop/params.py
index 9add5a8..f7c287c 100644
--- a/secop/params.py
+++ b/secop/params.py
@@ -31,8 +31,6 @@ from secop.datatypes import BoolType, CommandType, DataType, \
 from secop.errors import BadValueError, ProgrammingError
 from secop.properties import HasProperties, Property
 
-UNSET = object()  # an argument not given, not even None
-
 
 class Accessible(HasProperties):
     """base class for Parameter and Command
diff --git a/secop/properties.py b/secop/properties.py
index 047c147..74ba68a 100644
--- a/secop/properties.py
+++ b/secop/properties.py
@@ -26,8 +26,11 @@
 import inspect
 
 from secop.errors import BadValueError, ConfigError, ProgrammingError
+from secop.lib import UniqueObject
 from secop.lib.py35compat import Object
 
+UNSET = UniqueObject('undefined value')  #: an unset value, not even None
+
 
 class HasDescriptors(Object):
     @classmethod
@@ -39,9 +42,6 @@ class HasDescriptors(Object):
             raise ProgrammingError('misplaced trailing comma after %s.%s' % (cls.__name__, '/'.join(bad)))
 
 
-UNSET = object()  # an unset value, not even None
-
-
 # storage for 'properties of a property'
 class Property:
     """base class holding info about a property