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variable: $yrot -- rotate brain around y-axis
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keyboard equivalent: [Cmd/Alt]-left/right arrow
direct manip. equiv.: Shift-left-click-drag-left/right

The "yrot:" entry (lower left) rotates brain
surface around a vertical axis (i.e., around the
y-axis) in degrees (positive rotates the right
side of the brain towards the viewer).

Clicking in the slider troughs rotates 1 degree.

In a tcl rendering script, this rotation can be
achieved with:

  rotate_brain_y <deg>
  redraw

If focus is on the brain surface window, the
left/right arrows rotate the brain -2/+2 degrees
(in the interface window, cmd-arrow is required
to avoid instead moving the cursor in an entry).
