Philippe Schnoebelen, 13 Apr 1995
Original problem 6638 from feenschach asks to add some given men to the diagram position so that we get a "white legal cluster". I know what is an illegal cluster but not what is a legal cluster. Who can help ?
A legal cluster is a legal position that becomes illegal if one removes one piece. I guess a white legal cluster is a legal position that becomes illegal if one removes a white piece.