George Duke wrote on Mon, Aug 10, 2009 10:18 PM UTC:
Another Robinson puzzle that current Alphanumeric belongs in the same class by being exchange of positions. And Robinson comment leads to Hippodrome on 4x4
http://www.chessvariants.org/solitaire.dir/hippodrome.html of Andy Lewicki
http://www.chessvariants.org/solitaire.dir/quadrille.html and her own Queen's Quadrille above. The potential threefold similarities among some combinations of these draw from small boards 4x4 to 6x6, piece occupancy over 60% or over 80%, and thirdly exchange of positions being the puzzle or win condition. By happenstance there are several of them around Karen Robinson. Alphanumeric at ChessboardMath8 has 100% piece density on 6x6 and two opposite goal squares by exchange of positions.
http://www.chessvariants.org/index/displaycomment.php?commentid=23477 Goal squares feature in Roberto Lavieri's Maxima for another case.