anchor.diff:    ///  * <https://docs.microsoft.com/en-us/typography/opentype/spec/features_ae#dnom>
     const FontFeature.denominator() : feature = 'dnom', value = 1;
   
  -  // frac
     /// Use ligatures to represent fractions. (`afrc`)
     ///
     /// When this feature is enabled (and the font supports it),
  
positive.code:   ///  * [FontFeature.superscripts], which is similar but for subscripting.
    const FontFeature.subscripts() : feature = 'subs', value = 1;
  
    // sups
    /// Enable superscripts. (`sups`)
    ///
    /// This feature causes some fonts to change some glyphs to their
  
negative.code:   ///
    /// The default behavior varies from font to font.
    ///
    /// {@tool sample}
    /// The Kufam font supports the `pnum` feature. It causes the digits
    /// to become proportionally-sized, rather than all being the same
    /// width. In this font this is especially noticeable with the digit
  