code: public void appendDouble(double x) {
          ByteArrayOutputStream buffer = new ByteArrayOutputStream(8);
          DataOutputStream bufout = new DataOutputStream(buffer);
          try {
              bufout.writeDouble(x);
              appendBytes(buffer.toByteArray(), 0, 8);
          } catch (IOException e) {
              throw new AssertionError("write");
          }
      }
docs: Append a double as a eight byte number.