 Note that Post’s reformulation of the Turing machine is very much rooted in his Post 1936. (Some of) Post’s modifications of Turing’s definition became part of the definition of the Turing machine in standard works such as Kleene 1952 and Davis 1958. Since that time, several (logically equivalent) definitions have been introduced. Today, standard definitions of Turing machines are, in some respects, closer to Post’s Turing machines than to Turing’s machines. In what follows we will use a variant on the standard definition from Minsky 1967 which uses the quintuple notation but has no E and F-squares and includes a special halting state H. It also has only two move operations, viz., L and R and so the action whereby the machine merely prints is not used. When the machine is started, the tape is blank except for some finite portion of the tape. Note that the blank square can also be represented as a square containing the symbol \(S_0\) or simply 0. The finite content of the tape will also be called the dataword on the tape.