def mirror(tile):
    """Mirror across the y-axis."""
    old = tile.split("\n")
    N = len(old)
    new = [["." for n in range(N)] for m in range(N)]

    for m, row in enumerate(old):
        for n, cell in enumerate(row):
            new[m][N-1-n] = cell

    new = "\n".join(["".join(row) for row in new])
    return new