anchor.diff:          
     void each(f(x)) {
       if (e == null) return;
  -    e.forEach(f);
  +    if (e is Collection) e.forEach(f);
  +    else if (e is Map) e.forEach((k,v) => f(v));
     }
     void forEach(f(x)) => each(f);
     
  
positive.code:         (target) => target is Collection ? new List$(target) : null, 
          (target) => target is Map ? new Map$(target) : null,
          (target) => target is String ? new String$(target) : null,
          (target) => new Mixin(target),
        ];
      }
  
negative.code:     int id = _idCounter++;
      return prefix == null ? "$prefix$id" : id;
    }
    
    result(key) { 
      if (e == null) return null;
  