add ()

parse.leg

@@ -309,7 +309,7 @@ postfix		= s:atom ( "?" -		{ s = mkQuery(s) }
 			 | "+" -		{ s = mkPlus(s) }
 			 )?			{ $$ = s }
 
-atom		= string | class | dot | rule
+atom		= string | class | dot | rule | '(' -expression')'-
 
 rule		= i:id !'='        		{ $$ = mkId(intern(yytext)) }