[plt-scheme] how to creating a new "primitve type"

Hi all -

say that I want to create a new primitive type (something like path or
string) so it has the first class status (for the lack of a better term) as
the other primitive types.  How is this done, and Is this doable only in
scheme or does it involve working on C code?

Example - social security number.

> (define s1 #ssn"123-45-6789")
> s1
#ssn"123-45-6789"
> (ssn->string s1)
"123-45-6789"
> (ssn? s1)
#t
> (equal? s1 #ssn"123-45-6789")
#t
> (equal? s1 #ssn"987-65-4321")
#f

Any pointer is appreciated ;)  Thanks,
yinso
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