String interpolation bug, libsass 3.3.3

SassMeister-input.scss

// ----
// libsass (v3.3.2)
// ----

@function slice($list, $start: 1, $end: length($list), $sep: list-separator($list)) {
  $output: ();
  @if $start >= 1 and $end >= $start {
    @for $i from $start through $end {
      $output: append($output, nth($list, $i), $sep);
    }
  }
  @return $output;
}

@function power-set($list) {
  $result: ((),);
  @for $i from 1 through length($list) {
    @for $j from 1 through length($result) {
      $result: append($result, (join(nth($result, $j), nth($list, $i))));
    }
  }
  @return slice($result,2);
}

@function to-string($list, $glue: '') {
  $result: '';
  $length: length($list);
  @for $n from 1 through $length {
    $result: $result + nth($list, $n) + if($n == $length,'',$glue);
  }
  @return quote($result);
}

@each $set in power-set(a b c d) {
  .wrap--#{to-string($set, '-')} {
    debug: 'hello';
  }
}

SassMeister-output.css

.wrap--a {
  debug: 'hello';
}

.wrap--b {
  debug: 'hello';
}

.wrap--a-b {
  debug: 'hello';
}

.wrap--c {
  debug: 'hello';
}

.wrap--a-c {
  debug: 'hello';
}

.wrap--b-c {
  debug: 'hello';
}

.wrap--a-b-c {
  debug: 'hello';
}

.wrap--d {
  debug: 'hello';
}

.wrap--a-d {
  debug: 'hello';
}

.wrap--b-d {
  debug: 'hello';
}

.wrap--a-b-d {
  debug: 'hello';
}

.wrap--c-d {
  debug: 'hello';
}

.wrap--a-c-d {
  debug: 'hello';
}

.wrap--b-c-d {
  debug: 'hello';
}

.wrap--a-b-c-d {
  debug: 'hello';
}