/usr/share/fish/completions/at.fish is in fish-common 2.4.0-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 | #at
complete -f -c at -s V --description "Display version and exit"
complete -f -c at -s q --description "Use specified queue"
complete -f -c at -s m --description "Send mail to user"
complete -c at -s f -x -a "(__fish_complete_suffix (commandline -ct) '' 'At job')" --description "Read job from file"
complete -f -c at -s l --description "Alias for atq"
complete -f -c at -s d --description "Alias for atrm"
complete -f -c at -s v --description "Show the time"
complete -f -c at -s c --description "Print the jobs listed"
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