Swift `in` keyword meaning?

Closure expression syntax has the following general form:

In a named function, we declare the parameters and return type in the func declaration line.

func say(s:String)->() {
    // body
}

In an anonymous function, there is no func declaration line - it's anonymous! So we do it with an in line at the start of the body instead.

{
    (s:String)->() in
    // body
}

(That is the full form of an anonymous function. But then Swift has a series of rules allowing the return type, the parameter types, and even the parameter names and the whole in line to be omitted under certain circumstances.)