code: public CollectionComposer<MapComposer<PARENT>,?> startArrayField(String fieldName)
      {
          _closeChild();
          _fieldName = fieldName;
          CollectionComposer<MapComposer<PARENT>,?> child = _startCollection(this);
          _map.put(fieldName, child._collection);
          return child;
      }
docs: /*
  ********************************************************************
  /* Compose methods, structures
  ********************************************************************