Failed in extending the operator in subclass

[yiakwy]

Hi, I am going to extend operators in the following manner:

export class Operator {

	__add__(other: VecInterface) {
		throw Error("__add__ not implemented yet!")
	}

	// $FlowFixType
	[Symbol.for('+')] (other: any) {
		return this.__add__(other)
	}
	
	__sub__(other: VecInterface) {
		throw Error("__sub__ not implemented yet!")
	}

	// $FlowFixType
	[Symbol.for('-')] (other: any) {
		return this.__sub__(other)
	}

}

// I use mixin because Point2D should inherit it from Geometry, and es6 is currently only supporting mono inheritance
mixin(Point2D.prototype, Operator.prototype) // like jquery `extend`

I tested it in runtime by executing

let a = new Point2D(1, 0)
let b = new Point2D(0, 1)
let c = a + b // VecInterface point to Vec2 or Vec3

But it add, and sub was not fired. Could you give me any suggestion about it?