Turns out, that _gnu_generic doesn't handle GNU tail's --help
output all that well. If someone wants completion for tail,
send a specialised _tail function to zsh's development list.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=780531 as well.
# use generic completion system for programs not yet defined; (_gnu_generic works
# with commands that provide a --help option with "standard" gnu-like output.)
for compcom in cp deborphan df feh fetchipac gpasswd head hnb ipacsum mv \
- pal stow tail uname ; do
+ pal stow uname ; do
[[ -z ${_comps[$compcom]} ]] && compdef _gnu_generic ${compcom}
done; unset compcom