[plt-scheme] evaluating buffer w/o re-initializing environment?

Dear PLT-Scheme'rs,

One of the things I really miss in Dr. Scheme is the ability to add
new definition bindings without losing the state of the global
environment.  When you evaluate a buffer with F5 or Run, the world is
reset and then the buffer definitions are evaluated and put back.

Am I missing something easy, or is this a deep design decision?  I
wish I could introduce my students to the Lisp programming methodology
of iteratively building up the objects in one's world and the
procedures for interacting with them.

Fred