anchor.diff:    IndexedValue(this.index, this.value);
   }
   
  -
   /**
    * An [Iterable] of [IndexedValue]s where the nth value holds the nth
    * element of [iterable] and its index. See [enumerate].
  
positive.code: 
  /** The [Iterator] returned by [EnumerateIterable.iterator]. */
  class EnumerateIterator<V> extends Iterator<IndexedValue<V>> {
    final Iterator<IndexedValue<V>> _iterator;
    int _index = 0;
    IndexedValue<V> _current;
  
  
negative.code: 
    EnumerateIterable(this._iterable);
  
    Iterator<IndexedValue<V>> get iterator =>
        new EnumerateIterator<V>(_iterable.iterator);
  
    // Length related functions are independent of the mapping.
    int get length => _iterable.length;
  