public BodyEnumerator(BodyCollection collection, List <Body> list)
 {
     this._collection      = collection;
     this._list            = list;
     this._generationStamp = collection._generationStamp;
     i = -1;
 }
 void IDisposable.Dispose()
 {
     _collection = null;
     _list       = null;
     i           = -1;
 }