Simple example using SwiftInSwift ( https://gist.github.com/anonymous/07d9df1a80820bb5abf5a2c671fd223f ), and some definitions of stuff not visible here, ie the +=| and «» operators, etc.

Vector.swift

// DEF-{

func _sisCodeForVectorProtocols() -> [String] {
    var lines = [String]()
    for c in 2 ... 4 {
        lines +=| "public protocol Vector\(c) : CustomStringConvertible {"
        lines +=| "    associatedtype Element"
        lines +=| "    var elements: («Element») { get set }" «» c
        lines +=| "    init(«_ §a: Element»)" «» c
        lines +=| "}"
    }
    for c in 2 ... 4 {
        lines +=| "public extension Vector\(c) {"
        lines +=| "    init(all value: Element) { self.init(«value») }" «» c
        lines +=| "    var description: String { return String(describing: elements) }"
        lines +=| "    var §a: Element { return elements.§N }" *| c
        lines +=| "}"
    }
    return lines
}

// }-DEF
// PRINT-{ _sisCodeForVectorProtocols()
public protocol Vector2 : CustomStringConvertible {
    associatedtype Element
    var elements: (Element, Element) { get set }
    init(_ a: Element, _ b: Element)
}
public protocol Vector3 : CustomStringConvertible {
    associatedtype Element
    var elements: (Element, Element, Element) { get set }
    init(_ a: Element, _ b: Element, _ c: Element)
}
public protocol Vector4 : CustomStringConvertible {
    associatedtype Element
    var elements: (Element, Element, Element, Element) { get set }
    init(_ a: Element, _ b: Element, _ c: Element, _ d: Element)
}
public extension Vector2 {
    init(all value: Element) { self.init(value, value) }
    var description: String { return String(describing: elements) }
    var a: Element { return elements.0 }
    var b: Element { return elements.1 }
}
public extension Vector3 {
    init(all value: Element) { self.init(value, value, value) }
    var description: String { return String(describing: elements) }
    var a: Element { return elements.0 }
    var b: Element { return elements.1 }
    var c: Element { return elements.2 }
}
public extension Vector4 {
    init(all value: Element) { self.init(value, value, value, value) }
    var description: String { return String(describing: elements) }
    var a: Element { return elements.0 }
    var b: Element { return elements.1 }
    var c: Element { return elements.2 }
    var d: Element { return elements.3 }
}
// }-PRINT