Definitions
Proca Retractor: In a Defensive Retractor, you retract both white and black moves. Black is defending by trying to retract the moves that will forbid (or delay) the achievement of White's goal. Proca retractor in 5 (say) asks for White to retract 5 moves and then mate in 1 with a forward move. After every White retraction (but the last) Black retracts what he wants, trying to defend against the forward mate
