# Get arg as a MVisibility
	#
	# Return `null` if no option with that `key` or if the value is not in
	# `allowed_visibility`.
	fun visibility_arg(key: String): nullable MVisibility do
		var value = string_arg(key)
		if value == null then return null
		if not allowed_visibility.keys.has(key) then return null
		return allowed_visibility[value]
	end