Compute the cells visible from both cell a and b.
V = sudoku_isvisiblefrom2 ( a , b )
a 1x2 matrix, where a(1) is a row index and a(2) is a column index
a 1x2 matrix, where b(1) is a row index and b(2) is a column index
a 9x9 boolean matrix. V(i,j) is true if the cell (i,j) is visible from both cell a and cell b.
Returns the matrix of cells which are visible from both the cells a and b. The cell p is visible from both cell a and b if : * a and b are in the same row and p is on this row, * a and b are in the same column and p is on this column, * p in the same row as a and on the same column as b, * p in the same column as a and on the same row as b, * p in the same box as a and on the same row as b, * p in the same box as a and on the same column as b, * p in the same row as a and on the same box as b, * p in the same column as a and on the same box as b, * p in the same box as a and b.