Class iterator

test/cfg/boost.cpp:214–223

    static const int arity = 1;

    struct iterator {
        int operator*() const { return b; }
        void operator++() {
            a = a + b;
            std::swap(a, b);
        }
    private:
        int a = 1;
        int b = 1; // b is the output
    };

    // size is infinite
    bdata::size_t size() const { return bdata::BOOST_TEST_DS_INFINITE_SIZE; }