Hiding the left pane while changing "span9" to "span12" for the right pane actually does the job.
In Split Pane.tersus, this is the display after you press the Soups menu item:
And this is the display after you press the Hide Menu button on the right of the header:
If you look at the modeling of the button, you'll see the usage of Add Style Class and Remove Style Class, as explained in http://www.tersus.com/?Id=6277.
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