backward(desired) {
		if (desired != null) {
			this.__l_out.toGradientVector(desired, this.out.w, this.out.dw) // convert 'desired' to target vector
		}
		
		this.propagate() // propagate errors backwards

		return this.loss(desired) // return loss
	}
Propagates error back, error is provided by subtracting desired from actual output values. 
@param  {Float64Array | Int} desired
@return {Float}         loss