Question on variance text

[choppsv1]

I'm reading through the subtyping and variance chapter and the following text confused me:

Also, we saw that it was not ok for us to treat
`&mut &'a U` as a subtype of `&mut &'b U`,
therefore we can say that `&mut T` is *invariant* over `T`

Why is U introduced in the premise here and then not used in the conclusion? Shouldn't the premise just use T again, or if for some reason no, shouldn't the conclusion at least use U?

[senekor]

Yeah, I think there's no reason for that. Using T consistently would be more appropriate.