Turing machine deciding L = { w : w has an even number of 1s }
States: qeven (parity of 1s seen is even) and qodd (odd).
On reading 1 the machine toggles its state; on 0 it stays. On the blank
symbol □ it accepts from qeven and rejects from qodd.
This is exactly the two-state decider for even parity described on the page.