@function ballistic-sum($list) {
  $result: 0;

  $unit: unit( nth($list, 1) );

  @each $item in $list {
    @if type-of($item) == number {

      @if unit($item) == $unit {
        $result: $result + $item;
      } @else {
        @return ballistic-warn("Sum cannot calculate lists with values that have varying units");
      }

    } @else {
      @return ballistic-warn("Sum cannot calculate lists that contain values other than numbers");
    }
  }

  @return $result;
}