[plt-scheme] (require mzscheme) issue

dumb question, but am using v372...

isn't doing (require mzscheme), even at the top level, supposed to prevent  
me from later redefining standard procs and so on?

ie, shouldn't doing the following:

(require mzscheme)
(define + -)

return an error?

Or have I misunderstood something?

Psy-Kosh