Need a reference to current editor instance

Hi.

I'm using multiple editors in same form.

Have created a custom button, using a fckplugin.js in the plugin dir.

here is the code that is triggered when user click on the custom button:

ExpandVertical.prototype.Execute = function()
{
window.parent.set_editor_height( WHAT_DO_I_PUT_HERE );
}

I need to return a reference to current editor which I can use in my set_editor_height function so I
can do something like this:

function set_editor_height(id){
document.getElementById(id).height = parseInt(document.getElementById(id).height) + 50;
}

what can I use instead of WHAT_DO_I_PUT_HERE in:
window.parent.set_editor_height( WHAT_DO_I_PUT_HERE );
to make it work?