Method change

example/E02-MockGUI.cpp:23–29

struct ProcessGParam : public GParam {
    CVoid change(const std::string& name, bool isBegin) {
        if (isBegin) {
            running_elements_.insert(name);
        } else {
            running_elements_.erase(name);
        }
    }

    CVoid print() {
        std::cout << "<";