# If needed, truncate `self` at `max_length` characters and append an informative `message`.
	#
	# ~~~
	# assert "hello".trunc(10) == "hello"
	# assert "hello".trunc(2) == "he[truncated. Full size is 5]"
	# assert "hello".trunc(2, "...") == "he..."
	# ~~~
	fun trunc(max_length: Int, message: nullable String): String
	do
		if length <= max_length then return self
		if message == null then message = "[truncated. Full size is {length}]"
		return substring(0, max_length) + message
	end