PEP 671: Mention PEP 661, which solves some of the same problems

pep-0671.rst

@@ -109,6 +109,20 @@ bound arguments are broadly equivalent to code at the top of the function::
         if target was omitted: target = []
 
 
+Interaction with other open PEPs
+================================
+
+PEP 661 attempts to solve one of the same problems as this does. It seeks to
+improve the documentation of sentinel values in default arguments, where this
+proposal seeks to remove the need for sentinels in many common cases. PEP 661
+is able to improve documentation in arbitrarily complicated functions (it
+cites ``traceback.print_exception`` as its primary motivation, which has two
+arguments which must both-or-neither be specified); on the other hand, many
+of the common cases would no longer need sentinels if the true default could
+be defined by the function. Additionally, dedicated sentinel objects can be
+used as dictionary lookup keys, where PEP 671 does not apply.
+
+
 Open Issues
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