5+10. Proca-retractor. Wh. retracts 7 moves and mates in 1
Retract -1. Kd2xBe1! e2-e1=B+ -2. Kc3-d2 e4xPd3 e.p.+ -3. d2-d4 e5-e4+ -4. Kd3xPc3 b4xPc3 e.p.+ -5. c2-c4 b5-b4+ -6. Kc4xRd3 c6xPb5+ -7. Kc5-c4 and forward 1. b6 mate.
"After" -6. Kc4xRd3, the position is:
White's captures are bQ and bN on f3 and g3, bQB at home. bBa6 is from a promotion on f1 or d1, so that the bl. Ps made 8 captures. The wPa2 did not leave its column and appear in this inventory. Similarly, the wPb2 did not leave its column and thus had just been captured on b5. So that -6 ... c6xPb5+ is indeed the only legal retraction. If White had retracted -4. Kc4-d3?? without uncapturing a R, then Black could have legally retracted e.g. -6 ... c6xBb5+! with no forward mate.