# Number of UTF-8 characters in `self` starting at `from`, for a length of `byte_length`
	fun utf8_length(from, byte_length: Int): Int is intern do
		var st = from
		var ln = 0
		while byte_length > 0 do
			while byte_length >= 4 do
				var i = fetch_4_chars(st)
				if i & 0x80808080u32 != 0u32 then break
				byte_length -= 4
				st += 4
				ln += 4
			end
			if byte_length == 0 then break
			var cln = length_of_char_at(st)
			st += cln
			ln += 1
			byte_length -= cln
		end
		return ln
	end