Method half

example/print_half.cpp:42–42

    static leaf::result<BigInt> fromString(const std::string& s) { return BigInt{s}; }
    explicit BigInt(const std::string&) { }
    BigInt half() const { return BigInt{""}; }
    friend std::ostream& operator<<(std::ostream& o, const BigInt&) { return o << "big int half"; }
};