Filling grids with (row + col) % n
Math on the row and column numbers makes patterns: (row + col) % 2 is a checkerboard, (col - row) % 4 is a staircase, and col % 4 repeats a row every 4 beats.
Challenge A:Staircase: drums = [kick, snare, hihat]. For r in range(3), c in range(4): if (c - r) % 4 == 0, call drums[r](c + 1).