(;GM[1]FF[4]SZ[19]
GN[Just before]
PC[http://senseis.xmp.net/?RetrogradeAnalysisProblem3/Attempts]AP[GoWiki:2009]
DT[2009-04-22]
C[Diagram from http://senseis.xmp.net/?RetrogradeAnalysisProblem3/Attempts

Just before]
PL[B]

AB[lo][mo][no][po][ro][kp][np][nq][kr][pr][qr][rr][ps][qs][rs]
AW[mp][mq][oq][pq][qq][rq][sq][mr][sr][ms][ss]

;B[nr]C[B1]MN[1]
;W[or]C[W2]MN[2]
;B[os]C[B3]MN[3]
;W[ns]C[W4]MN[4]

;
)
