[plt-scheme] Debugging in drscheme

(Just posted in comp.lang.scheme)

Vista 64, PLT-scheme 4.1.3

I'm having trouble debugging in drscheme.  Perhaps someone can point
me in the right direction. I'm unable to find a single, clear, "here is
what you do" description in any of the numerous bits of documentation. 
Almost too much documentation with many links everywhere else makes it
hard to keep a good snapshot in my head.

The trouble lies in the first few lines:

#lang scheme
(require srfi/1) ;for list stuff
(require srfi/27) ;for randomness

This works with mzscheme.exe interpreter, and in works the same if I
click "run" from the DrScheme gui environment.

However if I click "debug" from the DrScheme env. I get the following
error: expr-syntax-object-iterator: unknown expr: (#%variable-
reference)

If  I comment out the line (require srfi/1), then both drscheme env.
and mzscheme.exe run until it encounters the "delete" (list
manipulation) procedure from srfi 1 later on in the code.

I tried changing delete to use the remove procedure, but then that
uncovered other things that aren't in scheme/base (such as list-index
I think, and possibly other stuff, used elsewhere in code).  I suppose
there is a reason the list manipulation stuff shows up in the first
srfi...

If I change the first line to #lang r5rs, both drscheme and mzscheme
complain about "require".  Other errors happen when I try selecting
various languages r5rs, pretty big, teaching edditions, etc.

Anyway, I'm guessing there is a simple knob to tweak someplace in
drscheme which will enable the correct debugging of my code, while
also allowing me to use srfi/1 in both drscheme and mzscheme.  Can
someone please help me out?

thank you very much
michael