Gturing is a very simple turing machine simulator that takes a simple
ASCII file as its input:

  Any empty line or any line beginning with a hash (#) is ignored. In other
case, five tokens, each separated by a space, are expected. These are the
state number, the expected symbol, the symbol to write, the direction to
move, and the new state. Anything following these tokens is ignored.

  Expected symbols and symbols to write can be any printable character.
The blank character is represented by the underscore (_).  States are
integer numbers, starting from 0.  The possible directions to move are
left and right, represented by 'l' and 'r', respectively.

  The machine starts at state 0 and stops when it cannot find the new
state or the new state doesn't expect the read symbol.