intellipad crashes immediately with this mgrammar input. I'm guessing stack overflow.
module HelloWorld {
    language HelloWorld {
        syntax Main
         = List(Hello);
        token Hello
         = "Hello";
        syntax List(Content, Separator)
         = Content
         | List(Content,",") Separator Content;
        
        syntax List(Content) = List(Content, ",");
    }
}