Dynamic updating

[Bruce Butterfield <bab@entricom.com>]

In Kent Pitman's article:

http://world.std.com/~pitman/PS/Hindsight.html

he discusses (among other topics) one very interesting property of 
Common Lisp:

 >Lisp allows you to enter a running image, build up a lot of state by 
 >running programs, and then at that point load new definitions. This 
 >gives product maintainers the powerful ability to have customers load 
 >patches that can fix bugs in running programs without requiring those 
 >programs to be exited. The value to a customer in some situations can 
 >be immeasurable.

In hopes of not starting any language wars on this very high S/N mailing 
list I would like to know if this is a property of CL itself or is this 
possible in mzscheme as well?