redef fun to_a
	do
		mutex.lock
		var r = real_collection.to_a
		mutex.unlock
		return r
	end

	# Get an array of the contained elements
	# Order is preserved
	#
	#     var tree = new OrderedTree[Int]
	#     tree.add_all(null, [1, 2])
	#     tree.add_all(1, [11, 12])
	#     tree.add_all(11, [111, 112])
	#     tree.add_all(12, [121, 122])
	#     tree.add_all(2, [21, 22])
	#     assert tree.to_a == [1, 11, 111, 112, 12, 121, 122, 2, 21, 22]
	redef fun to_a: Array[E] do
		var res = new Array[E]
		for r in roots do sub_to_a(r, res)
		return res
	end