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I see in Essential Go that using a mutex within a struct is not too straight-forward. To quote from the Mutex Gotchas page:

Don’t copy mutexes

A copy of sync.Mutex variable starts with the same state as original mutex but it is not the same mutex.

It’s almost always a mistake to copy a sync.Mutex e.g. by passing it to another function or embedding it in a struct and making a copy of that struct.

If you want to share a mutex variable, pass it as a pointer *sync.Mutex.

I'm not quite sure I fully understand exactly what's written. I looked here but still wasn't totally clear.

Taking the Essential Go example of a Set, should I be using the mutex like this: