Method begin

tests/mathreference.h:90–90

    using Ref = testdatatype_for_function_t<F, T>;
    static const Ref begin_, end_;
    static const Ref *begin() { return &begin_; }
    static const Ref *end() { return &end_; }
    static std::size_t size() { return end() - begin(); }
    const Ref &operator[](std::size_t i) const { return begin()[i]; }