In the manual some files are mentioned like this one

You can find a portable implementation of this extended case in
compat/caseext.fs.

Where can I find this file? I searched my computer but could not find it.

I’m trying to use the case as mentioned on that page:

case
  condition1 ?of exit-code1 endof
  condition2 ?of exit-code2 endof
  condition3 ?of exit-code3 endof
  ...
next-case
common code afterwards