Working on dimensional analysis, I have some proof-of-concept code that seems to be working:
let n1 = kilogram * meter / second * second
([(kg⋅m) / s]⋅s)
let n2 = kilogram * meter / (second * second)
[(kg⋅m) / (s⋅s)]
Note: () around unit products,  around unit quotients.
I'd like to adjust the precedence of operator * for my Unit protocol to be higher than /. Is that possible? It wasn't at all clear to me how to do that in Swift 3, or if can even be done at all.