[plt-scheme] polymorphism of primitive types

What is "shared"?
It seems to be defined in (lib "shared.ss"), yet it is available
without requiring it.
Also, how come following code works?
;;; =============================
(module redefine-syntax mzscheme
  (provide (rename my-shared shared))
  (define my-shared 'different-shared))

(module foo mzscheme
  (require redefine-syntax)
  (display shared))

(require foo)
;;; =============================

Yoav