See:
* [vim the hard way](https://learnvimscriptthehardway.stevelosh.com/chapters/23.html)

## Defining functions
```vim
function CommentLine()
  :s@.*@<!--&-->@
endfunction
function UnCommentLine()
  :s@<!--\(.*\)-->@\1@
endfunction 
```

## Calling functions
```vim
call CommentLine()
```

## Mapping Functions
```vim
nnoremap \kc :call CommentLine()<cr>
vnoremap \kc :call CommentLine()<cr>
```