Swift automatically provides shorthand argument names to inline closures, which can be
used to refer to the values of the closure’s arguments by the names $0, $1, $2, and so on.
If you use these shorthand argument names within your closure expression, you can omit
the closure’s argument list from its definition, and the number and type of the shorthand
argument names will be inferred from the expected function type. The in keyword can also
be omitted, because the closure expression is made up entirely of its body: