state by undoing actionBecause of this method, it is important that any action can be undone knowing only the post-state and the action.
	# Modify `state` by undoing `action`
	# Because of this method, it is important that any action can be undone
	# knowing only the post-state and the action.
	fun backtrack(state: S, action: A) is abstract
					lib/ai/backtrack.nit:64,2--67,47