[racket] Constructors for sequences

On 2012-04-29 12:43:48 -0400, Matthias Felleisen wrote:
> What you're really saying is that sequence-map uses the wrong kind of type. Specifically, it should be polymorphic in the sequence constructor instead of mapping everything to the top of the class hierarchy (sequence).

I don't think this is just a type issue. The sequence that is returned
by `sequence-map` is lazy, even if the original sequence was not. That's
why you can't get the original type.

Cheers,
Asumu