diff --git a/Parser/Python.asdl b/Parser/Python.asdl
index d49138f..b0d383f 100644
--- a/Parser/Python.asdl
+++ b/Parser/Python.asdl
@@ -55,6 +55,7 @@
 	     | Dict(expr* keys, expr* values)
 	     | ListComp(expr elt, comprehension* generators)
 	     | GeneratorExp(expr elt, comprehension* generators)
+	     -- the grammar constrains where yield expressions can occur
 	     | Yield(expr? value)
 	     -- need sequences for compare to distinguish between
 	     -- x < 4 < 3 and (x < 4) < 3