1. Myhill.thy is proved to correct some typos. 2. Some sections are added to the first direction. 3. The small theory of list difference is now separated into Prefix_subtract.thy and the ROOT.ML

(*
  no_document use_thys ["This_Theory1", "This_Theory2"];
  use_thys ["That_Theory1", "That_Theory2", "That_Theory3"];
*)
  use_thys ["../Prefix_subtract", "../Myhill"];