Thanks! Precise as always.
I'm not sure if it's the best way to do so, but it worked for me by
using a lambda:
(setq citar-file-open-function '(lambda (file)
(async-shell-command (format-message "sioyek \"%s\"" file))))
Moreover, since I don't really care about the terminal output of the PDF
viewer, I followed an Emacs SE suggestion
(https://emacs.stackexchange.com/a/58341/29817) and made the output
buffer not brought up:
(add-to-list 'display-buffer-alist '("*Async Shell Command*"
display-buffer-no-window (nil)))
which, in case I want to inspect it, it's still visitable with the usual
C-x b.
Thank you!
Alessandro
