Use "root" for preprocessor

preprocessorRoot.scss

// SASS/SCSS, less or stylus
.children-class {
  color: red;

  .parent-class & {
    color: green;
  }
}
=> compile in CSS
.children-class { color: red }
.parent-class .children-class { color: green }

// Do not confuse with "at-root" of SASS/SCSS for BEM
.level-1 {
  color: red;

  @at-root #{&}__level-2 {
    color: green;
  }
}
=> compile in CSS
.level-1 { color: red }
.level-1__level-2 { color: green }