[plt-scheme] Major math screwiness going on with fractions!

Hi everyone,


I was tracking down a problem in a program I was running involving numeric 
comparisons.  I've isolated it down to this:

###############
dyoo at dyoo-desktop:~$ mzscheme
Welcome to MzScheme v369.6 [3m], Copyright (c) 2004-2007 PLT Scheme Inc.
> (< 1 (/ 1 4))
#t
###############


After recovering from laughter, I looked at it further.  This bug appears 
to affect both mzscheme and mzschemecgc.  It's not a JIT problem:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
dyoo at dyoo-desktop:~/local/plt-svn/src/mzscheme$ PLTNOMZJIT=1 mzscheme
Welcome to MzScheme v369.6 [3m], Copyright (c) 2004-2007 PLT Scheme Inc.
> (< 1 (/ 1 4))
#t
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


It appears to have to do with rationals: if I convert down to inexacts, 
things work out as expected.

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> (< 1 (exact->inexact (/ 1 4)))
#f
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


It's in the logic of rational_lt in src/mzscheme/rational.c; some of the 
fast-path logic looks very suspicious to me, but I haven't put my finger 
on it yet.  If I comment out the fast case logic, the bug's fixed, so I 
know there's some something funky going on in the block here:

   /* Avoid multiplication in simple cases: */
   if (scheme_bin_lt_eq(ra->num, rb->num)
       && scheme_bin_gt_eq(ra->num, rb->num)) {
     if (!or_eq) {
       if (scheme_rational_eq(a, b))
         return 0;
     }
     return 1;
   } else if (or_eq) {
     if (scheme_rational_eq(a, b))
       return 1;
   }

I've commented it out in my own repository.  I have to go grab dinner 
right now, but hopefully someone can fix this?  It looks like it got 
introduced yesterday or so in r5455.


Anyway, hope this helps!