SPROUT_CXX14_CONSTEXPR reference at(size_type n);
SPROUT_CONSTEXPR const_reference at(size_type n) const;
#include <sprout/string.hpp>
using namespace sprout;
SPROUT_STATIC_CONSTEXPR auto input = string<8>("homuhomu");
static_assert(input.at(4) == 'h', "an element at position 5 is h.");