Example of automatic detection in Elvish of options, their arguments (e.g. <style>, <branch>) and their descriptions using comp:extract-opts

gistfile1.txt

> git checkout -<tab>
 COMPLETING argument  
 --conflict <style> (conflict style (merge or diff3))                                  --theirs (checkout their version for unmerged files) 
 --detach (detach HEAD at named commit)                                                --track (set upstream info for new branch)           
 --force (force checkout (throw away local modifications))                             -2 (checkout our version for unmerged files)         
 --ignore-other-worktrees (do not check if another worktree is holding the given ref)  -3 (checkout their version for unmerged files)       
 --ignore-skip-worktree-bits (do not limit pathspecs to sparse entries only)           -B <branch> (create/reset and checkout a branch)     
 --merge (perform a 3-way merge with the new branch)                                   -b <branch> (create and checkout a new branch)       
 --orphan <new-branch> (new unparented branch)                                         -f (force checkout (throw away local modions)) 
 --ours (checkout our version for unmerged files)                                      -l (create reflog for new branch)                    
 --overwrite-ignore (update ignored files (default))                                   -m (perform a 3-way merge with the new branch)       
 --patch (select hunks interactively)                                                  -p (select hunks interactively)                      
 --progress (force progress reporting)                                                 -q (suppress progress reporting)                     
 --quiet (suppress progress reporting)                                                 -t (set upstream info for new branch)                
 --recurse-submodules <checkout> (control recursive updating of submodules)