anchor.diff:  
     /// Randomly picks an entry of `this`.
     Pair<K, V> pickRandomEntry([Random random]);
  +
  +  /// A strict (non-lazy) version of [:map:].
  +  PersistentMap strictMap(Pair f(Pair<K, V> pair));
  +
  +  /// A strict (non-lazy) version of [:where:].
  +  PersistentMap<K, V> strictWhere(bool f(Pair<K, V> pair));
   }
   
   /**
  
positive.code:   }
  
    /**
     * Returns a new map identical to `this` except that it binds [key] to
     * [value].
     *
  
negative.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));
  
    PersistentMap<K, V> strictWhere(bool f(Pair<K, V> pair)) =>
      new PersistentMap<K, V>.fromPairs(this.where(f));
  }
  