 /// An enumeration representing the kNumberCaseType features.
 public enum NumberCase: FontFeatureProvider {
     case upper, lower
-    public func featureSettings() -> (Int, Int) {
+    public func featureSettings() -> (type: Int, selector: Int) {
         switch self {
         case .upper:
-            return (kNumberCaseType, kUpperCaseNumbersSelector)
+            return (type: kNumberCaseType, selector: kUpperCaseNumbersSelector)
         case .lower:
-            return (kNumberCaseType, kLowerCaseNumbersSelector)
+            return (type: kNumberCaseType, selector: kLowerCaseNumbersSelector)
         }
     }
 }

    func featureAttribute() -> StyleAttributes {
        let featureSettings = self.featureSettings()
        return [
            BONFontFeatureTypeIdentifierKey: featureSettings.0,
            BONFontFeatureSelectorIdentifierKey: featureSettings.1
        ]
    }
}

/// An enumeration representing the kNumberSpacingType features.
public enum NumberSpacing: FontFeatureProvider {
    case monospaced, proportional
    public func featureSettings() -> (type: Int, selector: Int) {
        switch self {
        case .monospaced:
            return (type: kNumberSpacingType, selector: kMonospacedNumbersSelector)
        case .proportional:
            return (type: kNumberSpacingType, selector: kProportionalNumbersSelector)
        }
    }
}
