Trouble finding fsService api

WP7702, legato version 19.11.5

I’m trying to use file i/o and the legato docs describe using fsService.
I added for example:
helloWorld.helloComponent.le_fs → fsService.le_fs
to my .adef file. This generates an error: Client interface ‘le_fs’ not found in component ‘helloComponent’
That was expected, and I have been trying to find the right line to add to my .cdef file. Normally, one would add:
le_fs.api to the requires: api: section, similar to other services. That results in error: Couldn’t find file ‘le_fs.api’
I have spelunked around in {LEGATO_ROOT} in search of the le_fs.api but have not been able to find it.
Where would I find that api, or am I on the wrong track for adding file I/O to my app?

does this help?