def get_alternatives_for_square(board, nrow, ncolumn):
    """Return sequence of valid digits for square (nrow, ncolumn) in board."""
    def _box(idx, size=3):
        """Return indexes to cover a box (3x3 sub-matrix of a board)."""
        start = (idx // size) * size
        return range(start, start + size)
    nums_in_box = [board[r][c] for r in _box(nrow) for c in _box(ncolumn)]
    nums_in_row = [board[nrow][c] for c in range(9)]
    nums_in_column = [board[r][ncolumn] for r in range(9)]
    nums = nums_in_box + nums_in_row + nums_in_column
    return sorted(set(range(1, 9+1)) - set(nums))