.PS
  r = 0.2

  define(`node',`circle rad r sprintf("%g", $1)')

  define(`tree',`[ifelse(eval($1<=1),1,`node($2)',
      `tree(eval($1 - 1), int(($2)/10))
       tree(eval($1 - 1), $2 - 11) with .nw at last [].ne + (0.1,0)
       node($2) at (last [].x+2nd last [].x)/2,last [].n.y+0.25
       line from last circle.c to 2nd last [].n chop r chop 0
       line from last circle.c to last [].n chop r chop 0 ')
       ]')

  tree(4, 1000)

.PE