It was Bent Larsen who figured out that this pawn can be advanced without the necessity of preparing it with a7-a6.
1. d2d4  1... g8f6  2. c2c4  2... c7c6  3. b1c3  3... d7d5  4. e2e3  4... e7e6  5. g1f3  5... f8e7 A rare move. Van Foreest had prepared something... 6. f1d3  6... d5c4  7. d3c4 