template<class InputIterator>
constexpr typename iterator_traits<InputIterator>::value_type
reduce(InputIterator first, InputIterator last);
template<class ExecutionPolicy, class ForwardIterator>
typename iterator_traits<ForwardIterator>::value_type
reduce(ExecutionPolicy&& exec,
ForwardIterator first, ForwardIterator last);
template<class InputIterator, class T>
constexpr T reduce(InputIterator first, InputIterator last, T init);
template<class ExecutionPolicy, class ForwardIterator, class T>
T reduce(ExecutionPolicy&& exec,
ForwardIterator first, ForwardIterator last, T init);
template<class InputIterator, class T, class BinaryOperation>
constexpr T reduce(InputIterator first, InputIterator last, T init,
BinaryOperation binary_op);
template<class ExecutionPolicy, class ForwardIterator, class T, class BinaryOperation>
T reduce(ExecutionPolicy&& exec,
ForwardIterator first, ForwardIterator last, T init,
BinaryOperation binary_op);