procedure main()
   s1 := collate(&cset,&cset)
   s2 := collate(reverse(&cset),reverse(&cset))
   write(image(decollate(s1,0)))
   write(image(decollate(s1,1)))
   write(image(decollate(s2,1)))
   write(image(decollate(s2,0)))
end

procedure collate(s1,s2)
  local length, ltemp, rtemp, t
  static llabels, rlabels, clabels, blabels, half
  initial {
    llabels := "ab"
    rlabels := "cd"
    blabels := llabels || rlabels
    clabels := "acbd"
    half := 2
    ltemp := left(&cset,*&cset/2)
    rtemp := right(&cset,*&cset/2)
    clabels := collate(ltemp,rtemp)
    llabels := ltemp
    rlabels := rtemp
    blabels := string(&cset)
    half := *llabels
    }
   if *s1 > *s2 then {
      t := s1[*s2+1:0]
      s1 := s1[1:*s2+1]
      }
   else if *s2 > *s1 then {
      t := s2[*s1+1:0]
      s2 := s2[1:*s1+1]
      }
   else t := ""
  length := *s1
  if length <= half then
    return map(left(clabels,2*length),left(llabels,length) ||
      left(rlabels,length),s1 || s2) || t
  else
    return map(clabels,blabels,left(s1,half) || left(s2,half)) ||
      collate(right(s1,length-half),right(s2,length-half)) || t
end

procedure decollate(s,n)
   static dsize, image, object
   local ssize
   initial {
      image := collate(&cset[2:0],repl(&cset[1],*&cset-1))
      object := string(&cset)
      dsize := *image
      }
   n %:= 2
   ssize := *s
   if ssize + n <= dsize then
      return map(object[1+:(ssize+n)/2],image[(n+1)+:ssize],s)
   else
      return map(object[1+:(dsize-2)/2],image[(n+1)+:dsize-2],
         s[1+:(dsize-2)]) || decollate(s[dsize-1:0],n)
end