class InputStream{
	optional<i32> readI32();
	optional<u8> readU8();

	optional<string> readStringWithPrefixLength(){
		int len = try readI32();
		string s;
		for ( int i = 0; i < len; i++ ){
			u8 ch = try readU8();
			s += ch;
		}
		return s;
	}

	optional<string> readStringWithPrefixLength(){
		optional<i32> o_len = readI32();
		if ( !o_len ) return nullopt;
		int len = o_len.value;

		string s;
		for ( int i = 0; i < len; i++ ){
			optional<u8> o_ch = readU8();
			if ( !o_ch ) return nullopt;
			u8 ch = o_ch.value;
			s += ch;
		}
		return s;
	}
};