gui.MouseEvent.Type.BUTTON_DOWN ... but how do I check if the right button is down?

[runra]

HI All,
Please excuse the noob question -- i don't program gui's.

An example on this page details a mouse call-back to obtain coordinates from the vizualizer gui using callbacks:
http://www.open3d.org/docs/0.15.1/python_example/visualization/index.html

It has the following line:
if event.type == gui.MouseEvent.Type.BUTTON_DOWN and event.is_modifier_down( gui.KeyModifier.CTRL)

It works beautifully...Ctrl-left click works great.

But how would the line differ to use a Ctrl-right click of the mouse?

I see there is a open3d.visualization.gui.MouseButton RIGHT object...but I don't know how to use it in the above line.
I tried :
if event.type == gui.MouseEvent.Type.BUTTON_DOWN and gui.MouseButton == RIGHT and\ event.is_modifier_down(gui.KeyModifier.CTRL):

This simply indicates that gui doesn't have an attribute MouseButton.

Thanks for your help.