Generated by SassMeister.com.

SassMeister-input-HTML.html

  <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
      <li>4</li>
      <li>5</li>
      <li>6</li>
      <li>7</li>
      <li>8</li>
      <li>9</li>
</ul>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@mixin nth-child($values...){
  $sel:();
  @each $value in $values {
    $sel: append($sel, unquote('#{&}:nth-child(#{$value})'), comma); 
  }
  @at-root { 
    #{$sel}{ @content; } 
  }
}


ul {
  li {
      @include nth-child(4, 3n){
        background-color:tomato;
      }
  }
}

SassMeister-output.css

ul li ul li:nth-child(4), ul li ul li:nth-child(3n) {
  background-color: tomato;
}

SassMeister-rendered.html

  <ul>
      <li>1</li>
      <li>2</li>
      <li>3</li>
      <li>4</li>
      <li>5</li>
      <li>6</li>
      <li>7</li>
      <li>8</li>
      <li>9</li>
</ul>