select2/docs/themes/learn2/scss/vendor/bourbon/css3/_font-feature-settings.scss

// Font feature settings mixin and property default.
// Examples: @include font-feature-settings("liga");
//           @include font-feature-settings("lnum" false);
//           @include font-feature-settings("pnum" 1, "kern" 0);
//           @include font-feature-settings("ss01", "ss02");

@mixin font-feature-settings($settings...) {
  @if length($settings) == 0 { $settings: none; }
  @include prefixer(font-feature-settings, $settings, webkit moz ms spec);
}