@function _is-context-within-selector($selector) {
  @if $selector == null { @return true; }
  @if & == null { @return false; }

  $selector-list: nth(selector-parse($selector), 1);
  $context-list: nth(selector-parse(&), 1);
  @for $i from 1 through length($selector) {
    @if nth($selector, $i) != nth($context-list, $i) {
      @return false;
    }
  }

  @return true;
}