protocol P {
    func f() -> Int
}

extension P {
    func f() -> Int { return 0 }
    func g() -> Int { return f() }
}

class X : P {
    init() {  }
}

class Y : X {
    // Compiler lets me write this, and claims that Y conforms to P (see
    // "redundant conformance" error below), however...
    func f() -> Int { return 1 }
}

print(Y().g()) // prints 0!

class Y1 : X {
    // error: method does not override any method from its superclass
    override func f() -> Int { return 1 }
}

// error: redundant conformance of 'Y2' to protocol 'P'
class Y2 : X, P {
    func f() -> Int { return 1 }
}