See pg 6 of AI Memo 864, A Robust Layered Control System for a Mobile Robot
	The horizontal decomposition in vertical slices forms a chain through which information flows from the robot’s environment, via sensing, through the robot and back to the environment, via action
	Each piece must have one instance built for the robot to run at all.
	If you change a vertical piece you have to preserve the interface or propagate the change altering neighbours’ functionality
	In 80’s robots could not deliver realtime performance in a dynamic world with this decomp.