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
Directory [tphols-2011] is added to host the commented version of [Myhill.thy]. Directory [pres] is created to host a build session for logical image [ListP] which is essentially [Main] extended with [List_prefix.thy].