/in/foo.cc: In function 'Polynomial quick_mul(Polynomial, Polynomial, int&)':
/in/foo.cc:84:13: error: request for member 'a' in 'ans_array', which is of non-class type 'll [800007] {aka long long int [800007]}'
   ans_array.a[i] = a.a[i] * b.a[i] % mod;
             ^
/in/foo.cc:86:6: error: 'struct Polynomial' has no member named 'ans'
  ans.ans = ans_array;
      ^~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:99:4: error: no match for 'operator[]' (operand types are 'Polynomial' and 'int')
   a[i] = (i & 1 == 1) ? mod - inv[i] : inv[i];
    ^
/in/foo.cc:99:17: warning: suggest parentheses around comparison in operand of '&' [-Wparentheses]
   a[i] = (i & 1 == 1) ? mod - inv[i] : inv[i];
               ~~^~~~
/in/foo.cc:100:4: error: no match for 'operator[]' (operand types are 'Polynomial' and 'int')
   b[i] = quick_pow(i, n, mod) * inv[i] % mod;
    ^
/in/foo.cc:102:8: error: cannot convert 'Polynomial' to 'll* {aka long long int*}' in assignment
  a.a = a;
        ^
/in/foo.cc:103:8: error: cannot convert 'Polynomial' to 'll* {aka long long int*}' in assignment
  b.a = b;
        ^