Thank you, Alarik. That seems promising, but not doing what I need.
If we refer to this image,
those 3 lines only made Elements 3 and 4 disappear. When I used the identifier "sidebar" for another line, it hides the content under zone 6, but I was hoping it would hide zone 5 including zone 6, as if you clicked on that "X".
More importantly, if I try to use those as inline Javascript within the "script" tag, they have no effect! 
In the end, I would like to force hide all of 2-6, leaving only 1 visible.
Where could I find the identifier for those elements, because I've tried to find that, and failed on my own, digging into the MDN site.