1.d4 d5 2.e4 e5 3.exd5 f6

White can play:

4.Bb5 c6 5.Ba4,

4.c3 c6,

4.Nc3,

4.Bc4 Bb4 (or 4...Bg4) 5.c3 Be6 (or 5...Bxc3) 6.Bxe6 Nh6 7.Bxh6 Qe7 8.Qg4 exd4 9.Qe6 Qxe6 10.Na3 Kg8 11.Kc1.