[plt-scheme] 3.99.0.20-svn25mar2008 [3m]: spurious debugging output?

Hi all,

I've just downloaded the build of 3.99.0.20 from yesterday, and
it seems to print a large quantity of debugging information when
my program is run (from inside DrScheme).  Vis:

(list 'row (var row)) (lambda (exp) (if (and (pair? exp) (equal? (car exp) 'row)) (let ((exp653 (cdr exp))) (if
(and (pair? exp653) (null? (cdr exp653))) (begin (set! row642 (car exp653)) #t) #f)) #f)) ((#f .
#(struct:srcloc #<path:/Applications/PLT Scheme
v3.99.0.20/collects/mzlib/private/match/render-test-list-impl.ss> 291 49 12901 926)) (#f . #(struct:srcloc
#<path:/Applications/PLT Scheme v3.99.0.20/collects/mzlib/private/match/render-test-list-impl.ss> 272 25 11774
2514)) (emit . #(struct:srcloc #<path:/Applications/PLT Scheme
v3.99.0.20/collects/mzlib/private/match/emit-assm.scm> 31 2 1097 849)) (emit . #(struct:srcloc
#<path:/Applications/PLT Scheme v3.99.0.20/collects/mzlib/private/match/emit-assm.scm> 31 2 1097 849)) (emit .
#(struct:srcloc #<path:/Applications/PLT Scheme v3.99.0.20/collects/mzlib/private/match/emit-assm.scm> 31 2
1097 849)) (#f . #(struct:srcloc #<path:/Applications/PLT Scheme
v3.99.0.20/collects/mzlib/private/match/coupling-and-binding.scm> 81 18 3918 194)) (emit . #(struct:srcloc
#<path:/Applications/PLT Scheme v3.99.0.20/collects/mzlib/private/match/emit-assm.scm> 31 2 1097 849)) (#f .
#(struct:srcloc #<path:/Applications/PLT Scheme
v3.99.0.20/collects/mzlib/private/match/coupling-and-binding.scm> 81 18 3918 194)) (emit . #(struct:srcloc
#<path:/Applications/PLT Scheme v3.99.0.20/collects/mzlib/private/match/emit-assm.scm> 31 2 1097 849))
(gen-match . #f) (#f . #(struct:srcloc #<path:/Applications/PLT Scheme 

and on and on, before it eventually gets to the standard output
from my own code.

It wasn't doing this with 3.99.12, which is the last previous
version that I've been using.  I noticed that a new debug logo
came up on the splash screen, so I guess that there's something
new included.  Is there a control that I should frob to "off"?

[In case it matters, this is on a PPC Mac running OS-X 10.4.foo]

Cheers,

-- 
Andrew