 
     }
 
-    /// An enumeration representing the kVerticalPositionType features
+    /// A feature provider for changing the vertical position of characters.
     public enum VerticalPosition: FontFeatureProvider {
 
+        /// No vertical position adjustment is applied.
         case normal
+
+        /// Superscript (superior) glpyh variants are used, as in footnotes¹.
         case superscript
+
+        /// Subscript (inferior) glyph variants are used: vₑ.
         case `subscript`
+
+        /// Ordinal glyph variants are used, as in the common typesetting of 4th.
         case ordinals
+
+        /// Scientific inferior glyph variants are used: H₂O
         case scientificInferiors
 
         public func featureSettings() -> [(type: Int, selector: Int)] {


    }

    /// An enumeration representing the kNumberSpacingType features.
    public enum NumberSpacing: FontFeatureProvider {

        case monospaced
        case proportional

        public func featureSettings() -> [(type: Int, selector: Int)] {

        }
    }

    /// A feature provider for changing the number case, also known as figure style.
    public enum NumberCase: FontFeatureProvider {

        /// Uppercase numbers, also known as lining figures, are the same height
        /// as uppercase letters, and they do not extend below the baseline.
        case upper

        /// Lowercase numbers, also known as oldstyle figures, are similar in
        /// size and visual weight to lowercase letters, allowing them to
        /// blend in better in a block of text. They may have descenders
        /// which drop below the typographic baseline.
        case lower

        public func featureSettings() -> [(type: Int, selector: Int)] {
