function to_gray(sequence image)
sequence color
    for i=1 to length(image) do
        for j=1 to length(image[i]) do
            -- unpack color triple
            color = sq_div(sq_and_bits(image[i][j], {#FF0000,#FF00,#FF}),
                                                    {#010000,#0100,#01})
            image[i][j] = floor(0.2126*color[1] + 0.7152*color[2] + 0.0722*color[3])*#010101
        end for
    end for
    return image
end function

sequence img = read_ppm("Lena.ppm")
    img = to_gray(img)