Index: parse.y
===================================================================
--- parse.y	(revision 32466)
+++ parse.y	(working copy)
@@ -746,6 +746,7 @@
 %left  keyword_or keyword_and
 %right keyword_not
 %nonassoc keyword_defined
+%nonassoc keyword_in
 %right '=' tOP_ASGN
 %left modifier_rescue
 %right '?' ':'
@@ -2289,6 +2290,15 @@
 			$$ = dispatch3(binary, $1, ripper_intern("!~"), $3);
 		    %*/
 		    }
+		| arg keyword_in arg
+		    {
+		    /*%%%*/
+			$$ = call_bin_op($3, rb_intern("include?"), $1);
+		    /*%
+			$$ = dispatch3(call, $3, ripper_intern("in"), ripper_intern("include?"));
+			$$ = method_optarg($$, $1);
+		    %*/
+		    }
 		| '!' arg
 		    {
 		    /*%%%*/