// Returns last index of $value in $list
// -------------------------------------------------------------------------------
// @documentation http://sassylists.com/documentation/#last-index
// -------------------------------------------------------------------------------
// @example last-index(a b c a, a) => 4
// @example last-index(a b c, z)   => null
// -------------------------------------------------------------------------------
// @param $list [List] : list
// @param $value [Literal] : value to be searched for
// -------------------------------------------------------------------------------
// @return [Number] | null

@function last-index($list, $value) {

    @for $i from length($list) * -1 through -1 {
        $i: abs($i);
        @if nth($list, $i) == $value {
            @return $i;
        }
    }

    @return null;
}