Step 1: Pick any empty cell and identify the closed path leading to that cell. A closed path consists of horizontal and vertical lines leading from an empty cell back to itself (If assignments have been made correctly, the matrix has only one closed path for each empty cell.) In the closed path there can only be one empty cell that we are examining. The 90-degree turns must therefore occur at those places that meet this requirement. Two closed paths are identified. Closed path a is required to evaluate empty cell A-E; closed path b is required to evaluate empty cell A-F.