Position: 0

Read: 'a'
Position: 1

Seek: +1
Position: 2

Read: 'c'
Position: 3

Put back: 'X'
Position: 2

Read: 'X'
Position: 3

Read: '\n'
Position: 4

Read: eof
Position: 4

Put back: 'Y'
Position: 3

Seek: -2
Position: 1

Read: 'b'
Position: 2