// Creates node representing virus of given id by constructing it from one parent.
    // Throws VirusAlreadyCreated if virus of given id already exists.
    // Throws VirusNotFound if given parent doesn't exist.
    void create(typename Virus::id_type const &id, typename Virus::id_type const &parent_id)
    {
        if (exists(id)) {
            throw VirusAlreadyCreated();
        }

        if (!exists(parent_id)) {
            throw VirusNotFound();
        }

        // Creates new virus as a pair of id and smart pointer to its wrapper to be inserted to virus_map:
        std::pair<typename Virus::id_type, std::shared_ptr<VirusGenealogyWrapper<Virus>>>
            n(id, std::make_shared<VirusGenealogyWrapper<Virus> >(id));
        // Inserts new pair into virus_map;
        virus_map.insert(n); // strong guarantee, if fails, nothing changes

        // Inserts new parent into the copy of id's parents set
        std::set<std::shared_ptr<VirusGenealogyWrapper<Virus>>> parents(n.second->parents);
        parents.insert(virus_map.find(parent_id)->second);

        // Inserts new child into the copy of id's parent's children set:
        std::set<std::shared_ptr<VirusGenealogyWrapper<Virus>>> parent_children(virus_map.find(parent_id)->second->children);
        parent_children.insert(virus_map.find(id)->second);

        // If none of above threw the exception, it means real sets can now be updated by no-throw swap:
        n.second->parents.swap(parents);
        virus_map.find(parent_id)->second->children.swap(parent_children);

    }