V. Ropke

White to move and mate in 6. Can you tell what is so unusual about this puzzle?

What is so unusual about this puzzle is that there is only one possible income of the game. The moves are as follows:

1. d4 b5
2. d5 b4
3. axb4 a3
4. b5 a2
5. b6 a1
6. b7x