Mission Possible

First, I need to fill an $N \times M$ grid with various types of blocks, leaving no empty cells. Then I repeat the following procedure until it can no longer be performed.

• Blocks of the same type connected horizontally or vertically belong to the same chain. All chains of size 3 or greater explode and disappear. The cells they occupied become empty.
• If a block has an empty cell immediately below it, it moves down by one cell. This is repeated until no block has an empty cell beneath it. A gravity system, in other words.
• If any blocks remain on the grid, return to the beginning and repeat the procedure. However, there must be no blocks that remain unexploded no matter how many times the procedure is repeated.

The goal is to fill the grid with blocks so that the number of repetitions of the procedure is as large as possible. I see... The difficulty could certainly frustrate a player, but it hardly seems enough to cause a riot. How strange. Shall we give it a try?

There are $T$ test cases. The first line contains $T$. $(1 \le T \le 20)$

Each of the following test cases consists of one line containing integers $N$ and , separated by a space.

For each test case, print the number of repetitions of the procedure on the first line.

Then print the configuration of the grid on the next $N$ lines.

On each line, print a string of length $M$ consisting of uppercase English letters. Equal letters represent blocks of the same type.