# Output a message with `level` severity
	#
	# Only output messages with `level` severity greater than of equal to `self.level`.
	#
	# ~~~
	# var stderr = new StringWriter
	# var logger = new Logger(warn_level, stderr, null)
	#
	# # This message will be displayed:
	# assert logger.warn("This is a warning.")
	# assert stderr.to_s.trim.split("\n").last == "This is a warning."
	#
	# # This message will not:
	# assert not logger.info("This is some info.")
	# assert stderr.to_s.trim.split("\n").last == "This is a warning."
	# ~~~
	#
	# Each logger can be given a default formatter used to format the messages
	# before outputting them:
	#
	# ~~~
	# var formatter = new DefaultFormatter(no_color = true)
	# logger = new Logger(warn_level, stderr, formatter)
	# logger.warn("This is a warning.")
	# assert stderr.to_s.trim.split("\n").last == "Warning: This is a warning."
	# ~~~
	#
	# Optionally, a `Formatter` can be given to replace the `default_formatter`
	# used by default.
	#
	# ~~~
	# # Create a formatter with no default decorator
	# logger = new Logger(warn_level, stderr, null)
	#
	# # Display a message without any formatter
	# logger.warn("This is a warning.")
	# assert stderr.to_s.trim.split("\n").last == "This is a warning."
	#
	# # Display a message with a custom formatter
	# logger.warn("This is a warning.", formatter)
	# assert stderr.to_s.trim.split("\n").last == "Warning: This is a warning."
	# ~~~
	fun add(level: Int, message: Writable, formatter: nullable Formatter): Bool do
		var format = formatter or else default_formatter
		if format == null then
			return add_raw(level, message)
		end
		return add_raw(level, format.format(level, message))
	end