The letter was written on a typewriter which has a problem with the top row of letters - they did not print. The letter should have read:
I would like to complain about the top row of my typewriter.
These letters appear not to work correctly.
As you can see, these letters are missing from this document.
I would be very grateful if you could correct this problem forthwith.
Your objective is to place some diagonal mirrors into the grid.
If a ray of light is shone in to the grid from each of the letters, and allowed to bounce off the internal diagonal mirrors, each will exit the grid at the twin of the letter that it entered the grid. For example, a ray entering at either letter D will bounce off some mirrors and exit the grid at the other letter D.
Each row and each column will contain exactly two of the diagonal mirrors.