Does the iOS Speech API support grammar?

Question

I was investigating various Speech Recognition strategies and I liked the idea of grammars as defined in the Web Speech spec. It seems that if you can tell the speech recognition service that you expect “Yes” or “No”, the service could more reliably recognize a “Yes” as “Yes”, “No” as `No”, and hopefully also be able to say “it didn’t sound like either of those!”.

However, in SFSpeechRecognitionRequest, I only see taskHint with values from SFSpeechRecognitionTaskHint of confirmation, dictation, search, and unspecified.

I also see SFSpeechRecognitionRequest.contextualStrings, but it seems to be for a different purpose. I.e., I think I should put brands/trademark type things in there. Putting “Yes” and “No” in wouldn’t make those words any more likely to be selected because they already exist in the system dictionary (this is an assumption I’m making based on the little the documentation says).

Is a way with the API to do something more like grammars or, even more simply, just providing a list of expected phrases so that the speech recognition is more likely to come up with a result I expect instead of similar-sounding gibberish/homophones? Does contextualStrings perhaps increase the likelihood that the system chooses one of those strings instead of just expanding the system dictionary? Or maybe I’m taking the wrong approach and am supposed to enforce grammar on my own and enumerate over SFSpeechRecognitionResult.transcriptions until I find one matching an expected word?

Unfortunately, I can’t test these APIs myself; I am merely researching the viability of writing a native iOS app and do not have the necessary development environment.