how to debug completion or: how to change _arguments depending on subcommand (not fixed position)

[zsh hh <zshuser1010@xxxxxxxxx>]

I want to create a completion script for:

> program [OPTS] SUBCOMMAND [SUBCOMMAND-OPTS] [ARGS]

A common approach is:
  _arguments -C \
    '1: :cmds' \
    '*::arg:->args' \
  && ret=0

  case $state in
    (args)
       case $line[1] in
         (subcmd1|subcmd2)
           _message 'no more arguments' && ret=0
         ;;
         (subwithextraopts)
           _arguments ...
             && ret=0
         ;;
         (othersubwithextraopts)
           _arguments ...
             && ret=0
         ;;
 esac

esac

But that relies on subcommand being at $line[1] which is rarely the
case (at least in my application)


git is actually a good example:

git -TAB #tabs git options

git add -TAB #tabs git-add options

git --bare log -TAB #tabs git-log options (but log is not $line[1]

That leads to the idea using git completion as reference, which in
turn leads to the question: how to debug such a completion?

It is a very extensive...