PDB Server - Keyword Definition
Verteidigungsrückzüger, Typ Proca
Defensive Retractor, Type ProcaIn 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.
The paradigm example of a defensive retractor is the Proca retractor stipulation, named after Zeno Proca (1906-1936).
A 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. White retractions must end in a forward mate against any retractive defense by Black. As in direct problems, White is allowed to mate forward in 1 at any time (after a retraction) and this may happen if Black picks a poor defense. Also, Black may defend by mating White through a forward move if such an opportunity exists after one of his retractions.
Rétracteur défensif, Type Proca