swift method exchange only in extensions?

import Cocoa

class Thing:NSObject {
    func a() {
        println("a")
    }
}

extension Thing {
//    func a() {
//        println("a")
//    }
}

extension Thing {
    func b() {
        println("b")
    }
}


func exchangeMethods(classType:AnyClass, methodName:String, swizzleName:String) {
    let method: Method = class_getInstanceMethod(classType, Selector.convertFromStringLiteral(methodName))
    let swizzleMethod: Method = class_getInstanceMethod(classType, Selector.convertFromStringLiteral(swizzleName))
    assert(method != nil, "Unable to find method to replace")
    assert(swizzleMethod != nil, "Unable to find replacung method")
    method_exchangeImplementations(method, swizzleMethod)
}

let foo = Thing()

foo.a()
foo.b()

println("")
exchangeMethods(Thing.self, "a", "b")

foo.a()
foo.b()

/// Output
// a
// b
// 
// a
// a

/// Comment out the a() implementation in the class body and uncomment the a() implementation in the extension
/// Output
// a
// b
//
// b
// a