Unify the unspecified argument thing.
diff --git a/OpenSSL/SSL.py b/OpenSSL/SSL.py
index 1215526..6dba10c 100644
--- a/OpenSSL/SSL.py
+++ b/OpenSSL/SSL.py
@@ -14,13 +14,12 @@
     exception_from_error_queue as _exception_from_error_queue,
     native as _native,
     path_string as _path_string,
+    UNSPECIFIED as _UNSPECIFIED,
 )
 
 from OpenSSL.crypto import (
     FILETYPE_PEM, _PassphraseHelper, PKey, X509Name, X509, X509Store)
 
-_unspecified = object()
-
 try:
     _memoryview = memoryview
 except NameError:
@@ -559,7 +558,7 @@
             raise exception
 
 
-    def use_privatekey_file(self, keyfile, filetype=_unspecified):
+    def use_privatekey_file(self, keyfile, filetype=_UNSPECIFIED):
         """
         Load a private key from a file
 
@@ -570,7 +569,7 @@
         """
         keyfile = _path_string(keyfile)
 
-        if filetype is _unspecified:
+        if filetype is _UNSPECIFIED:
             filetype = FILETYPE_PEM
         elif not isinstance(filetype, integer_types):
             raise TypeError("filetype must be an integer")
diff --git a/OpenSSL/_util.py b/OpenSSL/_util.py
index 1eec17e..a4fe63a 100644
--- a/OpenSSL/_util.py
+++ b/OpenSSL/_util.py
@@ -93,3 +93,8 @@
 else:
     def byte_string(s):
         return s
+
+
+# A marker object to observe whether some optional arguments are passed any
+# value or not.
+UNSPECIFIED = object()
diff --git a/OpenSSL/crypto.py b/OpenSSL/crypto.py
index 53a8dfc..4f0e7f2 100644
--- a/OpenSSL/crypto.py
+++ b/OpenSSL/crypto.py
@@ -14,7 +14,9 @@
     lib as _lib,
     exception_from_error_queue as _exception_from_error_queue,
     byte_string as _byte_string,
-    native as _native)
+    native as _native,
+    UNSPECIFIED as _UNSPECIFIED,
+)
 
 FILETYPE_PEM = _lib.SSL_FILETYPE_PEM
 FILETYPE_ASN1 = _lib.SSL_FILETYPE_ASN1
@@ -25,10 +27,6 @@
 TYPE_RSA = _lib.EVP_PKEY_RSA
 TYPE_DSA = _lib.EVP_PKEY_DSA
 
-# A marker object to observe whether some optional arguments are passed any
-# value or not.
-_undefined = object()
-
 
 
 class Error(Exception):
@@ -1835,7 +1833,7 @@
 
 
     def export(self, cert, key, type=FILETYPE_PEM, days=100,
-               digest=_undefined):
+               digest=_UNSPECIFIED):
         """
         export a CRL as a string
 
@@ -1862,7 +1860,7 @@
         if not isinstance(type, int):
             raise TypeError("type must be an integer")
 
-        if digest is _undefined:
+        if digest is _UNSPECIFIED:
             _warn(
                 "The default message digest (md5) is deprecated.  "
                 "Pass the name of a message digest explicitly.",