<===> input.scss
$list: one foo three bar six seven;
$pos: set-nth($list, 2, two);
$neg: set-nth($pos, -3, four five);

.test {
  -positive: $pos;
  -negative: $neg;
}

<===> output.css
.test {
  -positive: one two three bar six seven;
  -negative: one two three four five six seven;
}