SCSS. Reference second level ascending selector

Update: better than Original

.a{
  $grandparent: &;

  &:hover{
    color: red;

    & #{$grandparent}__b {
      color: blue;
    }
  }
}

and Original:

@function r-pseudo($s) {
  $string: nth(nth($s, 1), 1);
    @return str-slice($string, 0, str-index($string, ':') - 1);
}

.a{
  &:hover{
    color: red;

    & #{r-pseudo(&)}__b {
      color: blue;
    }
  }
}

both generate

.a:hover {
  color: red;
}
.a:hover .a__b {
  color: blue;
}