[plt-scheme] read-special

Hi all,

I've been looking at readable-snip<%> and the manual page for snip%,  
which instructions "To define a class of snips that read specially  
with open-input-text-editor", and wondering:  when is this used, and  
is there a more precise definition of "special" that's appropriate here?

(I'm guessing from number-snip.ss and scheme.ss in framework/private/  
that it's used for creating expressions when the containing editor is  
parsed into an AST, but, well, that's a guess.)

Thanks,
Jordan