procedure main()
   local a
   a := list(4)
   a[1] := create compact("a","b",create |reads(),a[2])
   a[2] := create compact("b","c",a[1],a[3])
   a[3] := create compact("c","d",a[2],a[4])
   a[4] := create compact("d","e",a[3],&main)
   while writes(@a[4])
end
