Method is_zero

Math/Subset Sum Problem.cpp:145–148

Beware!! p[i] = k won't change the value of p.a[i]

  inline mint coef(const int i) const { return (i < a.size() && i >= 0) ? a[i]: mint(0); }
  mint operator[](const int i) const { return (i < a.size() && i >= 0) ? a[i]: mint(0); } //Beware!! p[i] = k won't change the value of p.a[i]
  bool is_zero() const {
    for (int i = 0; i < size(); i++) if (a[i] != 0) return 0;
    return 1;
  }   
  poly operator + (const poly &x) const {
    int n = max(size(), x.size());
    vector<mint> ans(n);