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| template<typename I > requires bidirectional_iterator<I>
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| constexpr I | operator() (I it) const | 
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| template<typename I > requires bidirectional_iterator<I>
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| constexpr I | operator() (I it, iter_difference_t< I > n) const | 
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| template<typename I > requires bidirectional_iterator<I>
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| constexpr I | operator() (I it, iter_difference_t< I > n, I bound) const | 
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