This is an example SudokuXV starting grid, and I have highlighted some cells that we can either fill in,
or insert some pencil marks. In the topmost cell, it is linked to a '4' by a 'X' symbol. The 'X' tells
us that these two cells must add to 10, so the highlighted cell must be a '6'.
Slightly lower we have another highlighted cell linked by a 'V' to a 1. These two cells must add to 5,
so the highlighted cell must be a 4.
At the bottom of the puzzle we have two highlighted cells linked with a 'V'. We know these two cells
must add to 5, and there's only two ways we can do that, either 1+4=5, or 2+3=5. Because of the 1s and
4s already in these two rows we know that we can't have the combination of 1 and 4 in these cells. We
can then fill in the pencil marks of 2 and 3 for these two cells.