# Concatenate and separate each elements with `separator`.
	#
	# Only concatenate if `separator == null`.
	#
	# ~~~
	# assert [1, 2, 3].join(":")    == "1:2:3"
	# assert [1..3].join(":")       == "1:2:3"
	# assert [1..3].join            == "123"
	# ~~~
	#
	# if `last_separator` is given, then it is used to separate the last element.
	#
	# ~~~
	# assert [1, 2, 3, 4].join(", ", " and ")    == "1, 2, 3 and 4"
	# ~~~
	fun join(separator: nullable Text, last_separator: nullable Text): String
	do
		if is_empty then return ""

		var s = new Buffer # Result

		# Concat first item
		var i = iterator
		var e = i.item
		if e != null then s.append(e.to_s)

		if last_separator == null then last_separator = separator

		# Concat other items
		i.next
		while i.is_ok do
			e = i.item
			i.next
			if i.is_ok then
				if separator != null then s.append(separator)
			else
				if last_separator != null then s.append(last_separator)
			end
			if e != null then s.append(e.to_s)
		end
		return s.to_s
	end