PEP 635: Add reference to DLS paper

pep-0635.rst

@@ -86,6 +86,9 @@ We believe that adding pattern matching to Python will enable Python
 users to write cleaner, more readable code for examples like those
 above, and many others.
 
+For a more academic approach to this proposal, see [1]_.
+
+
 Pattern Matching and OO
 -----------------------
 
@@ -310,8 +313,6 @@ each composite statement starts with an identifying keyword, ``case`` seemed to
 be most in line with Python's style and traditions.
 
 
-
-
 Match Semantics
 ~~~~~~~~~~~~~~~
 
@@ -580,7 +581,6 @@ Python).  Other use cases are better expressed using guards.
 In the end, it was decided that this would make the syntax more complex
 without adding a significant benefit.  It can always be added later.
 
-
 **Example** using the OR pattern::
 
   def simplify(expr):
@@ -1218,6 +1218,15 @@ Or you would combine these ideas to write ``Node(right=y)`` so as to require
 an instance of ``Node`` but only extract the value of the `right` attribute.
 
 
+References
+==========
+
+.. [1] Kohn et al., Dynamic Pattern Matching with Python
+   https://doi.org/10.1145/3426422.3426983
+   (Accepted by DLS 2020.  The link will go live after Nov. 17;
+   a preview PDF can be obtained from the first author.)
+
+
 Copyright
 =========