[xul-interop] Querry element from different window

Hello

I'm a bit desperate to solve, in my opinion a trivial thing.
I have a toolbar and a sidebar.

I want buttons from the toolbar affecting content of a sidebar.
My problem is that for example document.getElementById('id') does not retreive elements from other XUL files (it is logical). But how can I access such elements? It has to through "window" .. I just don't know how.

Imagine having following defined in the sidebar:
<command id="cmd_openhelp" oncommand="alert('Help!');"/>

And I want following button from the toolbar to call it:
<button label="do" oncommand="goDoAction(....... cmd_openhelp)" />

Any help is greatly appreciated!

Thank You and have a greatest day!

Tomas
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