[hellofunk]

Still new to Swift but I believe & is an explicit syntax necessary to make clear in the code that the function being called is mutating the argument, and thus the variable passed into that function could be mutated. It must be used wherever an argument is in/out. It makes mutation explicit both in the function declaration and also in the function call. Which is nice!

It's good for code readability but also prevents accidentally passing a variable to a function that could mutate it when you weren't expecting that, and vice-versa.