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class FilePtr {
public:
    FilePtr() { fp = nullptr; }
    FilePtr(FILE *fp_) { fp = fp_; }
    FilePtr & operator = (FILE *fp_) { assert(fp == nullptr); fp = fp_; return *this; }
    ~FilePtr() { if (fp != nullptr) fclose(fp); }
    operator FILE * () { return fp; }
private:
    FILE *fp;
};