Install this package:
emerge -a media-plugins/vapoursynth-neo_f3kdb
If the package is masked, you can unmask it using the autounmask tool or standard emerge options:
autounmask media-plugins/vapoursynth-neo_f3kdb
Or alternatively:
emerge --autounmask-write -a media-plugins/vapoursynth-neo_f3kdb
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