\startMPinclusions
  numeric lw; lw:= 1;
  numeric n; n:= 8;
  numeric u; u:= 1cm;
\stopMPinclusions

\starttext
Before:\\
\startMPcode
  pickup pencircle scaled lw;
  draw unitsquare xyscaled (n*u,u);  
\stopMPcode
\\
After:\\
\startMPcode
  path p,q;
  pickup pencircle scaled lw;
  p:= unitsquare scaled u;
  for i=0 upto n-1:
      draw p xshifted i*u;
  endfor;
  q:= boundingbox currentpicture;
  draw (urcorner(q) shifted(-.5lw,-.5lw)) -- (urcorner(q) shifted(-.5lw,-.5lw +1.5u));
  setbounds currentpicture to q;
\stopMPcode
\stoptext