 }
 
 extension SIMDLabelBundle: CustomStringConvertible {
+  /// A string representation of `SIMDLabelBundle`.
   public var description: String {
     var s = "["
     for (weightIndex, maskIndex) in zip(weights.indices, validWeightsMask.indices) {

    assertConsistent()
  }

  public mutating func merge(with other: Self) {
    self += other
    assertConsistent()
  }

  public static func += (lhs: inout Self, rhs: Float) {
    lhs.weights += rhs
    lhs.weights.replace(with: 0, where: .!SIMDMask(lhs.validWeightsMask))  // Reset to 0
    lhs.assertConsistent()
  }

  public static func += (lhs: inout Self, rhs: Self) {
    lhs.assertConsistent()
    rhs.assertConsistent()

    lhs.assertConsistent()
  }

  /// Divide weights in `lhs` by the weights for the corresponding labels in `rhs`.
  ///
  /// - Precondition: `rhs` must have defined weights for every label defined in `lhs`.
  public static func / (lhs: Self, rhs: Self) -> Self {
    assert(lhs.validWeightsMask == lhs.validWeightsMask & rhs.validWeightsMask)
    let weights = lhs.weights / rhs.weights
    return Self(weights: weights, validWeightsMask: lhs.validWeightsMask)
  }

  /// Asserts that internal invariants hold true.
  func assertConsistent(file: StaticString = #file, line: UInt = #line) {
    assert(weights.replacing(with: 0, where: SIMDMask(validWeightsMask)) == .zero,
      """
