anchor.diff:      Sequential sequence();
       static Sequential $sequence([final Iterable $this]) => (() {
           Array array = new Array($this);
  -        return (() {
  -            if (array.empty) {
  -                return $package$empty;
  -            } else {
  -                return new ArraySequence(array);
  -            }
  -        })();
  +        return array.empty ? $package$empty : new ArraySequence(array);
       })();
       $dart$core.Object indexes();
       static $dart$core.Object $indexes([final Iterable $this]) => $package$measure(Integer.instance(0), $this.size);
  
positive.code:     }
      $dart$core.int get lastIndex => $outer$ceylon$language$Sequence.lastIndex;
      $dart$core.int size;
      $dart$core.Object getFromFirst([$dart$core.int index]) => (() {
          if ((index >= 0) && (index < size)) {
              return $capture$Sequence$collect$collecting.f(Sequence.$_$getElement($outer$ceylon$language$Sequence, index));
          } else {
              return null;
          }
      })();
      $dart$core.bool operator ==($dart$core.Object that) => List.$equals(this, that);
      $dart$core.int get hashCode => List.$get$hash(this);
      $dart$core.String toString() => Collection.$get$string(this);
  
negative.code:     Tuple slice([$dart$core.int index]);
      static Tuple $slice([final Sequence $this, $dart$core.int index]) => new Tuple.$withList([$this.spanTo(Integer.instance(index - 1)), $this.spanFrom(Integer.instance(index))]);
      Sequential measure([Integer from, $dart$core.int length]);
      static Sequential $measure([final Sequence $this, Integer from, $dart$core.int length]) => length > 0 ? $this.span(from, Integer.instance((Integer.nativeValue(from) + length) - 1)) : $package$empty;
      Sequential span([Integer from, Integer to]);
      static Sequential $span([final Sequence $this, Integer from, Integer to]) {
          if (Integer.nativeValue(from) <= Integer.nativeValue(to)) {
              return (Integer.nativeValue(to) >= 0) && (Integer.nativeValue(from) <= $this.lastIndex) ? new ArraySequence(new Array($this.sublist(Integer.nativeValue(from), Integer.nativeValue(to)))) : $package$empty;
          } else {
              return (Integer.nativeValue(from) >= 0) && (Integer.nativeValue(to) <= $this.lastIndex) ? new ArraySequence(new Array($this.sublist(Integer.nativeValue(to), Integer.nativeValue(from)).reversed)) : $package$empty;
          }
      }
      Sequential spanFrom([Integer from]);
  