[metapost] buildcycle

[js stock]

Dear MetaPost users,

  I'm having troubles with the buildcycle macro. I tried to look around, but
found nothing helpful. It's just a simple source where I'd like the
intersection of the two cycles to be filled. It seems nothing is filled, but
actually (you can see it if you comment the "draw p; draw q;" line), a very
tiny portion of the figure seems to be returned by "buildcycle".
Any help would be much appreciated!

GalTrack

##
beginfig(8);
numeric a,b,c,d;
a=.5in; b=.7in; c=.3in; d=.5in;
z1=-z3=(a,0); z2=-z4=(0,b);
path p,q;
p=z1..z2..z3..z4..cycle;
q=p shifted (-1cm,0);
draw p;
draw q;
fill buildcycle(p,q) withcolor black;
endfig;
end
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