how to subtype a handle by contents of handle

[lynch@ils.nwu.edu (Richard Lynch)]

Ok, maybe I'm just being stupid about how to do this, or maybe it's just
an inherently bad idea, but for some time now, I've been considering
writing a macro that do some fancy type-like stuff for handle-based data
structures.

Basically, I want to store a type-byte in the first byte of a handle, so I
want something like (def-datatype foo) to:
   (defun foop (x)
      (and (handlep x)
           (>= (#_GetHandleSize x) 1)
           (= (%hget-byte x) <auto-increment-counter>)))

    (deftype foo '(satisfies foop))

Now I realize that there is a chance that some arbitrary handle [say a
PICT] would be misconstrued as a foo, but I'm not real worried about it. 
I'm more interested in a good way to type-check among my own data-types.

My first attempts at writing def-datatype are failing miserably.

Any suggestions?

-- 
-- 
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-- "TANSTAAFL"  Rich lynch@ils.nwu.edu