ConstIterator& operator++() { ++I; return *this; };
bool operator==(const ConstIterator& X) const { return I == X.I; }
bool operator!=(const ConstIterator& X) const { return I != X.I; }
};
ConstIterator& operator++() { ++I; return *this; };
bool operator==(const ConstIterator& X) const { return I == X.I; }
bool operator!=(const ConstIterator& X) const { return I != X.I; }
};