This issue is now addressed by revision rev#83614

For now, we raise an error if the key is not PROCESS or THREAD or OBJECT.
In the future, when we implement support for free once key (needed to be able to reset a once, according to the specification), we will see how to handle the "typo" issue.

I think we should accept free once key only in addition to existing PROCESS, THREAD, OBJECT keys.

Then
  foo: FOO once ("PROCESS", "foo") ...
  foo: FOO once ("PROCESS", "foo", "bar") ...

would be valid but not
  foo: FOO once ("foo") ...
  foo: FOO once ("foo", "bar") ...

and obviously not the typo
  foo: FOO once ("PROCCCESSSSS") ...
  foo: FOO once ("PROCCCESSSSS", "bar") ...

This would be a nice way to detect typo on the 3 predefined keys
and still allow free keys

But for now, this is not specified (this way).

However, the next release will raise error if the once has free key(s).