  Morpion Solitaire is a pencil-and-paper game for a single player. A move in
this game consists of putting a cross at a lattice point and then drawing a
line segment that passes through exactly five consecutive crosses. The
objective is to make as many moves as possible, starting from a standard
initial configuration of crosses. For one of the variants of this game, called
5D, we prove an upper bound of 121 on the number of moves. This is done by
introducing line-based analysis, and improves the known upper bound of 138
obtained by potential-based analysis.
