What you’d really like to do in read-frame is read one byte and return **NIL** if it’s a null and otherwise read a frame with read-value. Unfortunately, if you read the byte in read-frame, then it won’t be available to be read by read-value.6

You can start by defining a condition type to be signaled by the low-level code and handled by the high-level code. This condition doesn’t need any slots—you just need a distinct class of condition so you know no other code will be signaling or handling it.

If you redefine id3-frame to make the type of its id slot frame-id instead of iso-8859-1-string, the condition will be signaled whenever id3-frame‘s method reads a null byte instead of the beginning of a frame.

With read-frame defined, you can now read a complete version 2.2 ID3 tag, representing frames with instances of generic-frame. In the “What Frames Do You Actually Need?” section, you’ll do some experiments at the REPL to determine what frame classes you need to implement. But first let’s add support for version 2.3 ID3 tags.