# Format `self` by replacing each `%n` with the `n`th item of `args`
	#
	# The character `%` followed by something other than a number are left as is.
	# To represent a `%` followed by a number, double the `%`, as in `%%7`.
	#
	# ~~~
	# assert "This %0 is a %1.".format("String", "formatted String") == "This String is a formatted String."
	# assert "Do not escape % nor %%1".format("unused") == "Do not escape % nor %1"
	# ~~~
	fun format(args: Object...): String do
		var s = new Array[Text]
		var curr_st = 0
		var i = 0
		while i < length do
			if self[i] == '%' then
				var fmt_st = i
				i += 1
				var ciph_st = i
				while i < length and self[i].is_numeric do
					i += 1
				end

				var ciph_len = i - ciph_st
				if ciph_len == 0 then
					# What follows '%' is not a number.
					s.push substring(curr_st, i - curr_st)
					if i < length and self[i] == '%' then
						# Skip the next `%`
						i += 1
					end
					curr_st = i
					continue
				end

				var arg_index = substring(ciph_st, ciph_len).to_i
				if arg_index >= args.length then continue

				s.push substring(curr_st, fmt_st - curr_st)
				s.push args[arg_index].to_s

				curr_st = i
				i -= 1
			end
			i += 1
		end
		s.push substring(curr_st, length - curr_st)
		return s.plain_to_s
	end