diff --git a/pep-3131.txt b/pep-3131.txt
index a2bc68a07..364259a2a 100644
--- a/pep-3131.txt
+++ b/pep-3131.txt
@@ -120,10 +120,30 @@ The following changes will need to be made to the parser:
    (such as pydoc) must be verified to continue to work when Unicode
    strings appear in ``__dict__`` slots as keys.
 
+Open Issues
+===========
+
+John Nagle suggested consideration of Unicode Technical Standard #39,
+[2]_, which discusses security mechanisms for Unicode identifiers.
+It's not clear how that can precisely apply to this PEP; possible
+consequences are
+
+ * warn about characters listed as "restricted" in xidmodifications.txt
+ * warn about identifiers using mixed scripts
+ * somehow perform Confusable Detection
+
+In the latter two approaches, it's not clear how precisely the
+algorithm should work. For mixed scripts, certain kinds of mixing
+should probably allowed - are these the "Common" and "Inherited"
+scripts mentioned in section 5? For Confusable Detection, it seems one
+needs two identifiers to compare them for confusion - is it possible
+to somehow apply it to a single identifier only, and warn?
+
 References
 ==========
 
 .. [1] http://www.unicode.org/reports/tr31/
+.. [2] http://www.unicode.org/reports/tr39/
 
 
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