Getting selection from instance of FCKeditor

How can I get the selected text from a specific instance of FCKeditor? Here is the context:

<form action="editor.php" method="post">

<script type="text/javascript">
<!--
var oFCKeditor = new FCKeditor("foobar");
oFCKeditor.BasePath = "/lt-fun/FCKeditor/";
oFCKeditor.Create();
//-->
</script>

<input type="button" value="Extract Selection"
onclick="extract(oFCKeditor)" />

</form>

<script type="text/javascript">
<!--
function extract(f)
{
// How to get the selection in f?
}
//-->
</script>

I'm looking for code that will work even when a page creates more than one instance of FCKeditor.

Also, I would like to know if this is documented somewhere on the FCKeditor site. Thanks.