Method move_back

smallthinker/common/chat-parser.h:42–47

        pos_ = pos;
    }
    void move_back(size_t n) {
        if (pos_ < n) {
            throw std::runtime_error("Can't move back that far!");
        }
        pos_ -= n;
    }

    // Get the substring of the input at the given range
    std::string str(const common_string_range & rng) const;