anchor.diff:    }
   
     /**
  -   * Creates a [PersistentMap] from an Iterable of Pairs using the default
  +   * Creates a [PersistentMap] from an [Iterable] of [Pair]s using the default
      * implementation of [PersistentMap].
      */
     factory PersistentMap.fromPairs(Iterable<Pair<K, V>> pairs) {
  
positive.code:   Option<V> lookup(K key);
  
    /**
     * Syntactic sugar for [lookup]
     */
    Option<V> operator [](K key) => this.lookup(key);
  
    /**
     * Evaluates `f(key, value)` for each (`key`, `value`) pair in `this`.
  
negative.code:   Pair<K, V> pickRandomEntry([Random random]) =>
        elementAt((random != null ? random : _random).nextInt(this.length));
  
    V operator [](K key) => this.lookup(key).asNullable;
  
    PersistentMap strictMap(Pair f(Pair<K, V> pair)) =>
      new PersistentMap.fromPairs(this.map(f));
  
  