[GiNaC-list] ex thread safe?

We are trying to implement the GiNaC calculation in multi-thread by using functor, auto func = [](void)->void {…}; However we found that (pointer) problem arises if the arguments are passed by reference. auto func = [&](void)->void {…}; But it seems okay to pass by value. auto func = [=](void)->void {…}; We were wondering if it is really okay to pass by value, or we have to transfer GiNaC::ex into string as argument. And we do not understand why passing by reference is not okay. Cheers, Zhao