[plt-scheme] A new version of Typed Scheme

Hi,

I've read the new Guide and find it much more helpful than the original one (which left me with many questions I must admit :-) )

Just one thing (hopefully it doesn't sound too stupid):
When reading, what surprised me was to find that e.g. True and False are both types, every symbol is a type... then I continued, reading about union types and subtyping, and now I'd be curious to ask if the whole type system is implemented as a hierarchy of unions, so something like 

(define-struct: Boolean (U True False))

would be the scheme way of doing Haskell's 

data Bool = False | True

In general, I wonder would it be possible to add a short introduction regarding concepts / implementation (for sure a detailed explanation of the concepts behind typed scheme do not belong in a guide, but ....)

Ciao,
Sigrid