lst-0698-book.cpp
// https://godbolt.org/z/nj14e6Wq4
#include <set>
#include <iostream>
int main() {
std::set<int> numbers{ 10, 20, 90 };
auto nein = numbers.find(30);
if(nein == numbers.end()) { std::cout << "nicht da.\n"; }
auto ja = numbers.find(20);
if(ja != numbers.end()) { std::cout << *ja << '\n'; }
}