Consider the problem of constructing crossword puzzles: fitting words into a grid of intersecting rows and columns of squares. Assume that a list of words (i.e. dictionary) is provided, and that the task is to fill in the rows and columns with words from this list so that if a row intersects with a column, their intersecting square has the same letter. Formulate this problem as an assignment (constraint satisfaction) problem by specifying the variables, their domains of possible values, and the constraints.