From profile , you can: place a vertical domino (stay at ), place two horizontal dominoes stacked (stay at ), place a tromino in row (move to ), or place a tromino in row (move to ).
From profile , you can: place a horizontal domino in row (move to ), or place a tromino covering row 's protruding cell and extending row (move to ).
From , transitions are symmetric. These define formulas: let be ways with profile , for , for . Then , , .