It had sonar distance and odometry on board, and used an offboard lisp machine to simulate subsumption architecture.
It used following internal representation, and every sonar return represented a repulsive force with, and inverse square drop off in strength.
Direction of its move was obtained by sum of the repulsive forces (suitably thresholded).
Additionally, the halt reflex of the lower level operated autonomously and unchanged.
The desired heading was then fed into a vector addition with the instinctive obstacle avoidance layer.