Panda puzzle P65199

Consider the puzzle to the right. We only have the sixteen 2 × 2 pieces shown, which we can place and rotate as we wish, as long as the board gets totally covered. There is just one restriction: Adjacent pieces must share the colors at the edges (white with white, black with black).

The puzzle has more than 40 thousand solutions. To order them, we construct a word from every solution, and we sort the words alphabetically. Suppose that we visit a solution by rows, from top to bottom, and each row from left to right. If we write an ‘a’ for every white square, and a ‘b’ for every black square, we get a word with 64 chars. For the solution in the image, the word is