Method extend

Math/Reeds Sloane Algorithm.cpp:12–15

struct LinearRecurrence {
  using vec = vector<ll>;
  static void extend(vec &a, size_t d, ll value = 0) {
    if (d <= a.size()) return;
    a.resize(d, value);
  }
  static vec BerlekampMassey(const vec &s, ll mod) {
    function<ll(ll)> inverse = [&](ll a) {
      return a == 1 ? 1 : (ll)(mod - mod / a) * inverse(mod % a) % mod;