histCollection::histDir_t Node1 histCollection::histDir_t + histDir_t() Node2 std::vector< histCollection ::histPerDir_t > Node2->Node1 +histos Node3 histCollection::histPerDir_t + mergeMethod + histPerDir_t() Node3->Node2 +elements Node4 std::string Node4->Node3 +name Node5 std::basic_string< char > Node5->Node4 Node6 TObject Node6->Node3 +obj Node7 TTree Node7->Node1 +md