[metapost] fill bpath does not work

[j.romildo at gmail.com]

Hello.

I have a small metapost figure which is not built as I expect. The
metapost program is as follows:

   % test.mp

   prologues := 2;
   input boxes;

   beginfig(6)
     boxjoin(  a.e = b.w - (4mm,0) );
     boxit.a1("1");
     boxit.a2("2");
     boxit.a3("3");
     fill bpath a1 withcolor 0.8white;
     fill bpath a2 withcolor white;
     fill bpath a3 withcolor 0.8white;
     drawboxed(a1,a2,a3);
   endfig;

   end

And the output image can be seen at
http://www.decom.ufop.br/prof/romildo/misc/test.6, while the the figure
I was expecting should look like this
http://www.decom.ufop.br/prof/romildo/misc/test.new.6

Can anybody take a quick look at this and tell me why that happens?

Regards,

Romildo