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v2 (2010-08-16):
  * NMI handler no longer uses the Y register without saving it.
  * Lots more comments in the NMI handler.
  * Vertical mirroring to illustrate how this effect would fit into a
    real game.
  * Reads the controllers during a period that is guaranteed
    glitch-free, then lets the player move the camera.
  * Actually sets the sample address so that the length-17 loop
    isn't playing garbage.
  * Priming playback no longer raises an IRQ.

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v1 (2010-08-12):
  * initial release