///Sums up all numeric values in `$list`, stripping unit if `$force` set to `true`.
///@ignore Documentation: http://sassylists.com/documentation.html#sl-sum
///@param {List} $list          - list
///@param {Bool} $force (false) - enable/disable parseInt
///@requires sl-every
///@requires sl-is-number
///@throws All items from list are not numbers for `sl-sum`.
///@example
///   sl-sum(1 2 3)
///   //6
///@example
///   sl-sum(a b 1)
///   //null
///@example
///   sl-sum(1 2 3px, true)
///   //6
///
///@return {Number}

 
@function sl-sum($list, $force: false) {
  $result: 0;

  @if not sl-every($list, sl-is-number) {
    @warn "All items from list are not numbers for `sl-sum`.";
    @return null;
  }

  @each $item in $list {
    @if not unitless($item) and $force {
      $item: $item / ($item * 0 + 1);
    }

    @if unitless($item) {
      $result: $result + $item;
    }
  }

  @return $result;
}