[plt-scheme] Parallel execution

I've been working my way through SICP and am currently in chapter 3.  
Right now, I'm reading about parallel execution. The syntax they use is

(parallel-execute <thunk> <thunk> ...)

I'm not sure how to do the same thing in PLT Scheme, but I did try

(begin (thread (lambda () (display "A"))) (thread (lambda () (display  
"B"))))

and sometimes get "AB" and sometimes "BA" (as I had hoped). So, how  
do begin and thread interact? I had expected that I'd see "AB" much  
more often than "BA" because I thought each thread would be scheduled  
as soon a (thread <thunk>) was evaluated. But now, I wonder how  
thread is implemented, and if none of the threads would actually run  
until the begin block was evaluated. Then I began to wonder how if  
there was just one "engine" (for lack of a better term) carrying out  
a single reduction in each thread and then moving on to another  
(round robin scheduling?)

Just wondering.

===
Gregory Woodhouse
gregory.woodhouse at sbcglobal.net

"The whole of science is nothing more than a refinement
  of everyday thinking."  -- Albert Einstein