A Turing Machine that decides 02n

M2 = "On input string w:
  1. Sweep left to right across the tape, crossing off every other 0.

  2. If in stage 1 the tape contained a single 0, accept.

  3. If in stage 1 the tape contained more than a single 0 and the number of 0's was odd, reject.

  4. Return the head to the left-hand side of the tape.

  5. Go to stage 1.