# Is `self` a valid UTF-8 sequence start ?
	#
	# ~~~nit
	# assert 0.is_valid_utf8_start
	# assert 0xC0.is_valid_utf8_start
	# assert 0xE0.is_valid_utf8_start
	# assert 0xF0.is_valid_utf8_start
	# ~~~
	fun is_valid_utf8_start: Bool do
		if self & 0x80 == 0 then return true
		if self & 0b1110_0000 == 0b1100_0000 then return true
		if self & 0b1111_0000 == 0b1110_0000 then return true
		if self & 0b1111_1000 == 0b1111_0000 then return true
		return false
	end