.PS

thicklines_
define(`HOMELIB_',`/u/aplevich/lib')
ifdef(`dline',,`include(HOMELIB_/darrow.m4)')

dx = 0.1
circlerad = boxht*3/8
extlen=4*dx
boxwid = 0.75
indel = boxht/3
eps=0.01
fillval = 0.8

COb: box fill_ "C$\bar{\mbox{O}}$"
CO:box fill_ with .nw at last box.se+(dx,-dx) "CO"
CbOb: box fill_ with .nw at last box.se+(dx,-dx) \
   "$\bar{\mbox{C}}$$\bar{\mbox{O}}$"
CbO: box fill_ with .nw at last box.se+(dx,-dx) "$\bar{\mbox{C}}$O"

   dline(from CbO.s down_ dx,,t)
   dright
{Sum: circle at (CO,Here)}
   darrow(to Sum.e)

SW: (COb.w,Sum.s)+(-3*dx-extlen/2,-dx)
NE: (CbO.e,COb.n)+(dx,dx)
   right_; shadebox(box wid NE.x-SW.x ht NE.y-SW.y with .sw at SW)

U: COb.w-(3*dx+extlen,0)
   right_; "\bf u$\,$" rjust at U; {box invis fill_(1) ht dlinewid wid extlen}
   dend ; dline(right extlen+dx,,t)
TU: dtee(`up_')
   {darrow(to (COb.w,Here),t,)}
   dline(to (Here,CO),t,t)
{Dum: circle at (Here,Sum)}
   down_; dtee(`left_')
   {darrow(to CO.w,t,)}
{D: box "{\bf D}" wid boxwid/2 ht boxwid/2 at 0.5<Here,Dum.n>}
   darrow(to D.n,t,)
   darrow(from D.s to Dum.n)

   dline(from CO.n to (CO,COb)-(0,indel),,t)
   up_; dleft; darrow(to (COb.e,Here))

   dline(from CbOb.n to (CbOb,COb),,t)
   up_; dleft; darrow(to COb.e)

   dline(from CbO.n-(indel,0) up_ boxht/2+dx,,t)
   dleft; darrow(to CbOb.e)

   dline(from CbO.n to (CbO,CO),,t)
   up_; dleft
   {box invis fill_(1) ht dlinewid wid boxwid+2*dx with .e at Here-(dx,0)}
   darrow(to CO.e)

   dline(from CbO.n+(indel,0) to (CbO,COb)+(indel,indel),,t)
   up_; dleft; darrow(to (COb.e,Here))

   darrow(from CO.s to Sum.n)

   move to Sum.w+(eps,0); left_
   {{line invis from Dum.w to (U,Dum)}
      box invis fill_(1) ht dlinewid wid last line.start.x-last line.end.x\
      with .e at Here-(dx,0)}
   darrow(to Dum.e)
   {box invis fill_(1) ht dlinewid wid dlinewid at (SW,Here)}
   darrow(from Dum.w to (U,Dum))
      {"{\bf y}$\,$" rjust }
   {line invis from Here+(-0.23,0) to Here+(-0.23,0.1)}

.PE