Issue #2335: Backport set literals syntax from Python 3.x.
diff --git a/Parser/Python.asdl b/Parser/Python.asdl
index f791709..ee6e999 100644
--- a/Parser/Python.asdl
+++ b/Parser/Python.asdl
@@ -56,6 +56,7 @@
 	     | Lambda(arguments args, expr body)
 	     | IfExp(expr test, expr body, expr orelse)
 	     | Dict(expr* keys, expr* values)
+	     | Set(expr* elts)
 	     | ListComp(expr elt, comprehension* generators)
 	     | GeneratorExp(expr elt, comprehension* generators)
 	     -- the grammar constrains where yield expressions can occur