I see the same as @mhvk re: `to` - it does what you'd expect.

I'm -0 on the inverse trig function returning `Angle`s - I'm thinking of `Angle`s as something of a convinience class - promoting `Quantity`s from regular math operations is starting a precedent that we always have to promote to the most-apropriate class, and I'd rather generally leave that to the user.  But I also realize that the result of an inverse trig function is physically an Angle, so it's not a terribly strong opinion if someone wants to implement it.