SPROUT_CONSTEXPR basic_string substr(size_type pos = 0, size_type n = npos) const;
#include <sprout/string.hpp>
#include <sprout/assert.hpp>
using namespace sprout;
SPROUT_STATIC_CONSTEXPR auto x = string<8>("homuhomu");
SPROUT_STATIC_CONSTEXPR auto y = x.substr(4, 4);
SPROUT_ASSERT_MASG(y == "homu", "y is substring of x.");