model appears as an essay What is a computation? in Steen pages 241–267. For some reason Davis has renamed his model a "Turing–Post machine" (with one back-sliding on page 256.) In the following model, Davis assigns the numbers "1" to Post's "mark/slash" and "0" to the blank square. To quote Davis: "We are now ready to introduce the Turing–Post Programming Language. In this language there are seven kinds of instructions: "PRINT 1 "PRINT 0 "GO RIGHT "GO LEFT "GO TO STEP i IF 1 IS SCANNED "GO TO STEP i IF 0 IS SCANNED "STOP "A Turing–Post program is then a list of instructions, each of which is of one of these seven kinds. Of course, in an actual program, the letter i in a step of either the fifth or sixth kind must be replaced with a definite (positive whole) number." (Davis in Steen, p. 247). 1994 (2nd edition): Davis–Sigal–Weyuker's Post–Turing program model "Although the formulation of Turing we have presented is closer in spirit to that originally given by Emil Post, it was Turing's analysis of the computation that has made this formulation seem so appropriate. This language has played a fundamental role in theoretical computer science." (Davis et al. (1994) p. 129) This model allows for the printing of multiple symbols. The model allows for B (blank) instead of S0. The tape is infinite in both directions. Either the head or the tape moves, but their definitions of RIGHT and LEFT always specify the same outcome in either case (Turing used the same convention). PRINT σ ;Replace scanned symbol with σ IF σ GOTO L ;IF scanned symbol is σ THEN goto "the first" instruction labelled L RIGHT ;Scan square immediately right of the square currently scanned LEFT ;Scan square immediately left of the square currently scanned This model reduces to the binary { 0, 1 } versions presented above, as shown here: PRINT 0 = ERASE ;Replace scanned symbol with 0 = B = BLANK PRINT 1 ;Replace scanned symbol with 1 IF 0 GOTO L ;IF scanned symbol is 0 THEN goto "the first" instruction labelled L IF 1 GOTO L ;IF scanned symbol is 1 THEN goto "the first" instruction labelled L RIGHT ;Scan square immediately right of the square currently scanned LEFT ;Scan square immediately left of the square currently scanned Examples of the Post–Turing machine Atomizing Turing quintuples into a sequence of