public protocol Layer: Differentiable {
  /// The input type of the layer.
  associatedtype Input: TensorGroup & Differentiable
  /// The output type of the layer.
  associatedtype Output: TensorGroup & Differentiable

  /// Returns the output obtained from applying to an input.
  @differentiable(wrt: (self, .0))
  func applied(to input: Input) -> Output
}

struct Classifier : Layer {
  var w1 = Tensor<Float>(randomUniform: [2, 4])
  var w2 = Tensor<Float>(randomUniform: [4, 1])
  var b1 = Tensor<Float>(zeros: [1, 4])
  var b2 = Tensor<Float>(zeros: [1, 1])
  func applied(to input: Tensor<Float>) -> Tensor<Float> {
    let o1 = sigmoid(matmul(input, w1) + b1)
    return sigmoid(matmul(o1, w2) + b2)
  }
}