Non-working example at `https://legacy.imagemagick.org/Usage/masking/#bg_remove`

[Nick Graham]

Version: ImageMagick 6.9.10-23 Q16 x86_64 20190101 https://imagemagick.org, Ubuntu 20.04, x86_64

Example under https://legacy.imagemagick.org/Usage/masking/#bg_remove does not work.
Downloading the example images, I expect the second command

convert overlay_difference.png -channel RGBA -separate +channel \
          -evaluate-sequence add  -threshold 0   overlay_mask.png

to output the image shown at https://legacy.imagemagick.org/Usage/masking/overlay_mask.png . Instead, I get a pure-white image.
Editing the command to

convert overlay_difference.png -channel RGB -separate +channel \
          -evaluate-sequence add  -threshold 0   overlay_mask.png

generates the expected mask.

I cannot figure out where the source for the legacy Usage section of the website is hosted, but the corresponding page for the new site is found here: https://github.com/ImageMagick/Usage/blob/c6dc6493a7de33503a569f9236c2be2be3c64d38/masking/index.html#L2566

[Cristy]

We corrected the documentation as you recommended. Thanks.