prologues := 3;
input boxes

beginfig(1);
arrowlenx=16mm; arrowleny=10mm;
pickup pencircle scaled 1pt;
circleit.xa(btex ${\bf x}_{k-1}$ etex scaled 1.5);
circleit.xb(btex ${\bf x}_k$ etex scaled 1.5);
boxit.za(btex ${\bf z}_{k-1}$ etex scaled 1.5);
boxit.zb(btex ${\bf z}_k$ etex scaled 1.5);
xa.dx = xb.dx = za.dx = zb.dx = defaultdx*4;
xa.dy = xb.dy = za.dy = zb.dy = defaultdy*4;
xpart xa.w = arrowlenx;
ypart za.s = ypart zb.s = 0;
xb.w = xa.e + (arrowlenx,0);
xa.s = za.n + (0,arrowleny);
xb.s = zb.n + (0,whatever);
drawboxed(xa, xb, za, zb);
drawarrow (xa.w - (arrowlenx,0))--xa.w;
drawarrow xa.e--xb.w;
drawarrow xb.e--(xb.e + (arrowlenx,0));
drawarrow xa.s--za.n;
drawarrow xb.s--zb.n;
endfig;
end