# Get the single argument of `self` as a `String`.
	# Raise error and return null on any inconsistency.
	fun arg_as_string(modelbuilder: ModelBuilder): nullable String
	do
		var args = n_args
		if args.length == 1 then
			var arg = args.first.as_string
			if arg != null then return arg
		end

		modelbuilder.error(self, "Syntax Error: `{name}` expects a single String as argument.")
		return null
	end