The main algorithm is to traverse the destination hierarchy. For each file in the destination, check to see if a corresponding file exists, if not then delete the destination file. Sound simple? Well, mostly it is given that Backup Folder already handles the traversal code.

set m_deleted_folder to true
...
  if (m_deleted_folder is true) then
    set m_deleted_folder to choose folder with prompt "Deleted items will be moved to this folder:"
  end if

First off, we have m_deleted_folder. If you try to delete a file on an AppleShare volume the Finder asks you if you want to delete it now, which hangs the AppleScript. So instead we can move the files to a folder and even set replacing true so that the file will always be moved there. Of course the folder still has to be emptied later. Maybe Mac OS X 10.1 allows you to always delete a file.

      -- See if the item exists at the destination
      try
        set l_source_item to get item (name of l_destination_item) of l_source_folder
      on error l_error
        set l_delete_item to true
      end try

I don't think I mentioned that it's easier to ask the Finder to get an item reference rather than ask it if an item exists. If you look at the Event Log, a command like if (exists file l_file of l_curr_folder) really does if (l_file is in (get every file of l_curr_folder)) which is kind of inefficient and might break if the folder has hundreds of files.

        -- Delete source 
        try
          if (class of m_deleted_folder is alias) then
            move l_destination_item to m_deleted_folder with replacing
          else
            delete l_destination_item
          end if
        on error l_error
          log "****" & l_error
        end try

This is the main file processing routine. Either we move the file to the deleted folder or try to delete the file. I also added error output which I think I forgot to do before. Notice that I use m_deleted_folder both to store a boolean value at the beginning (true or false indicating whether or not to ask for the folder) and the alias of the folder.

That's a strength and weakness of AppleScript -- no variable typing. You can assign any value type to a variable and then change the value type later with another assignment. You also don't initialize variables so if you go around changing a variable name throughout a script you might miss one and it's tough to catch. As you may guess it has happened to me. I guess I'm too used to strongly typed languages.

So now I have enough to properly synchronize one folder the same as another folder. Not very robust or perfect but now I can combine the last two scripts and have a credible backup solution. Next up is to combine the two scripts, add some initial processing (strictly for my web site), and actually log results to a file.