anchor.diff:    const _SimpleNumEnumerable(this._source) : super();
   
     Iterator<T> iterator() => _source.iterator();
  +
  +  int get length => count();
   }
   
   class _FuncNumEnumerable<TSource> extends NumberEnumerable {
  
positive.code:   const _FuncNumEnumerable(this._source, this._func) : super();
  
    Iterator<num> iterator() => _func(_source.iterator());
  }
  
  
  class _RangeIterable extends NumberEnumerable<int> {
    final int _start;
    final int _count;
  
    const _RangeIterable(this._start, this._count);
  
    Iterator<int> iterator() => new _RangeIterator(_start, _count);
  }
  
  class _RangeIterator implements Iterator<int> {
  
negative.code:   }
  
    factory NumberEnumerable.fromRange(int start, int count) {
      return new _RangeEnumerable(start, count);
    }
  
    num sum() {
      num theSum = 0;
  