It seems that this code can't be compiled with the current version of SObjectizer:

void so_define_agent() override {
  auto filter = [](const some_message &) { return true; };
  so_set_delivery_filter(mbox, filter);
}

because so_set_delivery_filter expects a rvalue reference to lambda-function.

Maybe it's necessary to fix so_set_delivery_filter to allow acceptance of lvalue references (const and non-const)?