function-token includes the opening (

Author: tabatkins

css-mixins-1/Overview.bs

@@ -953,7 +953,7 @@ and a [=mixin body=].
 save that it lacks a [=custom function/return type=].)
 
 <pre class="prod def">
-	<<@mixin>> = @mixin <<function-token>> ( <<function-parameter>># )?
+	<<@mixin>> = @mixin [<<function-token>> <<function-parameter>>#? ) | <<ident-token>> ]
 	{
 		<<declaration-rule-list>>
 	}