Clarify the existence of the <> operator in Grammar/Grammar with a comment, for issue 13239
diff --git a/Grammar/Grammar b/Grammar/Grammar
index 89f4c36..cea68de 100644
--- a/Grammar/Grammar
+++ b/Grammar/Grammar
@@ -88,6 +88,8 @@
and_test: not_test ('and' not_test)*
not_test: 'not' not_test | comparison
comparison: expr (comp_op expr)*
+# <> isn't actually a valid comparison operator in Python. It's here for the
+# sake of a __future__ import described in PEP 401
comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'
star_expr: '*' expr
expr: xor_expr ('|' xor_expr)*