python3 interpreter.py <program.ptm> --input <input>

This syntax tries to be as close as possible to the original Post-Turing machine, while still being programmable.

A program is a text file with one or more lines.

If there is a # symbol somewhere in the line, everything after it on that line (including the symbol itself) will be ignored.

A program is interpreted as a series of commands, commands are whitespace (or newline) separated.

Each command is a symbol 1, 0, <, >, ! or ?

There are 5 commands which take no arguments:

• 1 -- mark current cell (if it is already marked, nothing will happen)
• 0 -- erase the mark from current cell (if it isn't marked, nothing will happen)
• < -- move caret to the left
• > -- move caret to the right
• ! -- end the program immediately

And there is a command which takes 2 arguments, which must follow (whitespace or newline separated) immediately after the command itself.

• ? n m -- if there is a mark in current cell, go to $n$-th line of the program, otherwise go to the $m$-th line

The program starts execution at the first command (if there are no commands, it exits immediately).

After processing command !, program stops execution immediately.

After processing command 1, 0, < or >, the command that follows immediately after it is processed.

If no such command exists, the program ends execution (equivalent to !).

After processing command ? n m, if there is a mark in current cell, the first command in the part of a program starting from the $n$-th line is executed. Otherwise, the first command in the part of a program starting from the $m$-th line is executed.

If no such command exists, the program ends execution (equivalent to !).

Note: we denote a set mark as a 1, and unset mark as a 0.

Program can receive an input S -- binary string.

This means that marks to the right of the starting position are set according to the bits of S, with 0 encoded to 10 (N) and 1 encoded to 11 (Y) (e.g. input S = 0101 will set the marks to 10111011 with caret positioned at the first 1).

All the other marks' values are undetermined.

If a program exited, its output from a program is read in bit pairs from left to right, starting from the caret position and until receiving a 00 bit pair (F).

If a 01 bit pair (E) is read, the program is considered failed, and subsequent output will not be read.

Otherwise, the output is decoded back into a binary string with 10 (N) decoding to 0 and 11 (Y) decoding to 1.

For example, if after the program exited the values of the marks to the right of the caret (including the mark under the caret itself) are 1110111100..., the program's output will be interpreted as a binary string 1011.

Another example: an empty program outputs its input.

First of all, let's convert our program to a list of commands.

We can assign a number to each command sequentially, then add an argument to commands 1, 0, <, >, equal to the sequentially next number after the number of a current command.

? n m command can be replaced with ? N M, where $N$ and $M$ are, respectedly, numbers of first commands starting from $n$-th line and from $m$-th line.

Afterwards, we can replace commands which "jump" to non-existent command numbers with ! commands.

This will produce a correct Post-Turing machine program which will be equivalent to the original program in this syntax, as can be easily seen from the control flow rules.

First of all, we replace each command number $n$ with a placeholder $p_n$ that refers to the command that was $n$-th.

Then, we replace each command 1 n, 0 n, < n or > n with the command pairs (1, ? pn pn), (0, ? pn pn), (<, ? pn pn), (>, ? pn pn).

Afterwards, we put each command on a separate line and replace $p_n$ with the line number of a command that was originally $n$-th.

This is a correct program in our syntax, which produces equivalent results, as can be easily seen from the control flow rules.

QED.