     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
-    let validWeightsMask = lhs.validWeightsMask & rhs.validWeightsMask
-    return Self(weights: weights, validWeightsMask: validWeightsMask)
-  }
-
-  public static var zero: Self {
-    Self(weights: .zero, validWeightsMask: .zero)
-  }
-
+    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,
       """

    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()

    assertConsistent()
  }

  /// Sets weights for every label to `other`'s where the label is not defined in `self`, and is
  /// defined in `other`.
  public mutating func fillMissingFrom(_ other: Self) {
    let mask = (~validWeightsMask) & other.validWeightsMask
    weights.replace(with: other.weights, where: SIMDMask(mask))
