[racket] Use of "struct:"

Hi all,
    A simple question. I try typing the following example
(module nest racket
    (provide (struct-out egg))
    (struct egg (color wt)))

in this way
(module nest typed/racket
    (provide (struct-out egg))
    (struct: egg ([color : String]  [wt : Number])))

But I get:
   module: identifier already provided (as a different binding) in: egg

I disabled the compilation options in the "Choose Language" Dialog,
but I get the same error.

PS: Another simple question, Which is the syntax to define only one
mutable field in a "struct:" in Typed Racket?

Thank very much,
Paul,