Fish shell: Completion: how to define command sub-argument completion?

I would like if it is possible to define completion for sub arguments in fish Best explained w/ an example: $ somecmd -k foo[arg1,arg2,..argK,.,argN] -----------------------------^^^^ Cpmpletions for the arg1, arg2 ... argN ? Possible? How would you do it? The general interesting case for me is that argN takes it values in a finite list (depending on K and foo) N is variable I have imagined some things with matching list of regexes => tedious at best Have looked under /usr/local/share/fish/completions/*, found nothing resembling (ignoring the fact that '[' is a shell special char, in a first attempt ) Thanks very much in advance for any hints!