[plt-scheme] Can subclasses of editor-snips% be saved?

Hi everyone,

I've been trying to implement the serialization protocol for my 
editor-snip% subclass, but things aren't working so well.  I can see that 
my write method fires off, but when I try reading back, my snipclass is 
silent.

I've tried to condense my example as small as I can.  I've put the source 
of an 'my-editor-snip' collection here:

     http://hashcollision.org/tmp/my-editor-snip.tar.gz


I've tried to follow the docs on the protocol for serialization. I've 
implemented "write" for the snip, "read" for the snipclass, registered all 
snips to the snipclass, and registered the snipclass to 
(get-the-snip-class-list).

I must still be missing something: what am I missing?  Thanks for any 
help!