Mixin to insert class inside parent selector.

insert-parent.scss

@mixin insertParent($class, $append: false) {
  $selString: quote(&);
  $parent: str-slice($selString, 0, str-index($selString, " ") - 1);
  $minusParent: str-slice($selString, str-index($selString, " ") + 1);
  $firstChild: str-slice($minusParent, 0, str-index($minusParent, " ") - 1);
  $otherChildren: str-slice($selString, str-index($selString, $firstChild) + str-length($firstChild) + 1);
  
  $selector: unquote("");
  @if $append {
    $selector: $parent #{$firstChild}#{$class} $otherChildren;
  } @else {
    $selector: $parent $class $firstChild $otherChildren;
  }
  
  @at-root {
    #{$selector} {
      @content;
    }
  }
}