writeln into a textarea in a form

I have a page with a right and left frame and I would like buttons pushed on the left to write values/text into a textarea element in a form on the right frame.....I specifically need to have each button ADD text to the textarea, not replace it. Like building a shopping list or something like that. It seems that "writeln" is limited to writing only to the document object.

Here's the code I thought would work for the buttons on the left: onClick="parent.parent.RightFrame.document.form.textarea.writeln('Hello!');" ...but it doesn't work! Can you please help?

Thanks.

JG