# Is `self` a well-formed Base64 entity ?
	#
	# Will return an Error otherwise with info on which part is erroneous.
	fun check_base64(length: Int): nullable Error do
		var rlen = 0
		var opos = length
		for i in [0 .. length[ do
			if self[i] == u'=' then
				opos = i
				break
			end
			if self[i].is_whitespace then continue
			if not self[i].is_base64_char then return new Error("Invalid Base64 character at position {i}: {self[i].code_point}")
			rlen += 1
			if rlen > 4 then rlen -= 4
		end
		var pad = 0
		for i in [opos .. length[ do
			if self[i].is_whitespace then continue
			if self[i] != u'=' then return new Error("Invalid padding character {self[i].code_point} at position {i}")
			pad += 1
		end
		if rlen + pad != 4 then return new Error("Invalid padding length")
		return null
	end