The Cross Layout draws the focused window in the center of the screen
and part of the other windows on the sides. The Shrink and Expand
messages increment the size of the main window.
The focus keybindings change the center window, while other windows
cycle through the side positions. With the Cross layout only four
windows are shown around the focused window, two ups and two downs,
no matter how many are in the current stack. I.e. focus down cycles the
window below focused into the center; focus up cycles the window above.