##Extension A Swift extension is similar to an Objective-C category. Extensions expand the behavior of existing classes, structures, and enumerations, including those defined in Objective-C.
extension Double {
var km: Double { return self * 1_000.0 }
var m: Double { return self }
var cm: Double { return self / 100.0 }
var mm: Double { return self / 1_000.0 }
var ft: Double { return self / 3.28084 }
}