anchor.diff:  }
   
   /**
  - * An persistent map, binding keys of type [K] to values of type [V]. Null
  + * A persistent map, binding keys of type [K] to values of type [V]. Null
    * values are supported but null keys are not.
    *
    * Persistent data structure is an immutable structure, that provides effective
  
positive.code:   lookupIn(List path, {orElse()});
  
    /**
     * Returns the value for the given [key] or throws if [key]
     * is not in the map.
     */
    V operator [](K key);
  
  
negative.code: 
  part of persistent;
  
  /**
   * A read-only map, binding keys of type [K] to values of type [V]. Null
   * values are supported but null keys are not.
   *
   * There is no default implementation of [ReadMap], since it just
   * specifies common interface of [PersistentMap] and [TransientMap].
   * 
   * In all the examples below `{k1: v1, k2: v2, ...}` is a shorthand for
   * `new PersistentMap.fromMap({k1: v1, k2: v2, ...})`.
   */
  abstract class ReadMap<K, V> implements Iterable<Pair<K, V>> {
  
    /**
     * Returns the value bound to [key].
     * 
     * If [key] is not bound, [orElse] is called to obtain the
     * return value. Default [orElse] throws [RangeError].
     */
    V lookup(K key, {orElse()});
  
  