# Storage of nodes when `memorize` is activated
	# Each state is associated with a node.
	# This permit:
	#   * to avoid to revisit visited nodes (see `do_revisit`)
	#   * to avoid to reinvoke `heuristic` on known states (unless `memorize_late` is set)
	private var memory: Map[S, SearchNode[S, A]] = new HashMap[S, SearchNode[S, A]]