context: 1. d2d4  1... g8f6  2. c2c4  2... e7e6  3. g2g3  3... d7d5  4. f1g2  4... b8c6  5. g1f3  5... a8b8 A very rare move. According to my database, it is just the 10th choice for Black in this position. Caruana usually shows interesting ideas in the opening.