void abort() { };
                                                                                                                                                          
                                                                            

/*@ 
    requires (1);
    ensures ((\result == ((long long) n + m)));
@*/
int sum(int n, int m) {
    if (n <= 0) {
      return m + n;
    } else {
      return sum(n - 1, m + 1);
    }
}

int main() {
  int a = 20;
  int b = 0;
  int result = sum(a, b);
  if (result != a + b) {
    ERROR: {/*@ assert(0); */;abort();}
  }
}