[Retros] N-return discovered checkmate in 6.0

[Noam Elkies]

Meanwhile here's a captureless 6.0 move SPG (C+ Popeye in 0.1 sec.)
with 6...Nb8# discovering checkmate from the Bc8:

  1 d3 d5  2 Kd2 Qd6  3 Ke3 Qc5+  4 Kf4 Nd7  5 Kf5 f6  6 Ke6 Nb8#

NDE