[%c_effect c4;square;c4;type;Interesting;persistent;true]
1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. b1c3  3... f8b4  4. e2e3  4... e8g8  5. f1d3  5... c7c5  6. g1e2  6... b8c6  7. e1g1  7... d7d5  8. c4d5  8... e6d5  9. a2a3 A critical opening position.