TypeScript abstract method using lambda/arrow function

I am using TypeScript 1.6 and would like to create an abstract class with an abstract method but use a lambda/arrow function in the concrete class.

Is this possible? The code shown below does not compile as it says

"Class 'Base' defines instance member function 'def', but extended class 'Concrete' defines it as instance member property"...

abstract class Base {
    abstract abc(): void;
    abstract def(): void;
}

class Concrete extends Base {
    private setting: boolean;

    public abc(): void  {
        this.setting = true;
    }

    public def = (): void => {
        this.setting = false;
    }
}

My understanding of Typescript specifications is that when you are declaring

public def = (): void => {
    this.setting = false;
}

You are actually declaring a property called def and not a method on the Base class.

Properties cannot (unfortunately IMHO) be abstracted in Typescript: https://github.com/Microsoft/TypeScript/issues/4669