anchor.diff:    Option<V> lookup(K key);
   
     /**
  -   * Syntactic sugar for [lookup]
  -   */
  -  Option<V> operator [](K key) => this.lookup(key);
  +   * Returns the value for the given [key] or [:null:] if [key]
  +   * is not in the map.
  +   */
  +  V operator [](K key);
   
     /**
      * Evaluates `f(key, value)` for each (`key`, `value`) pair in `this`.
  
positive.code:   Pair<K, V> pickRandomEntry([Random random]) =>
        elementAt((random != null ? random : _random).nextInt(this.length));
  
    PersistentMap strictMap(Pair f(Pair<K, V> pair)) =>
      new PersistentMap.fromPairs(this.map(f));
  
  
negative.code:  */
  abstract class PersistentMapBase<K, V>
      extends IterableBase<Pair<K, V>>
      implements PersistentMap<K, V> {
  
    Map<K, V> toMap() {
      Map<K, V> result = new Map<K, V>();
  