/in/foo.cc:15:12: error: 'ostream' does not name a type
   15 |     friend ostream& operator <<(ostream& os, BigInt& b);
      |            ^~~~~~~
/in/foo.cc:16:12: error: 'istream' does not name a type
   16 |     friend istream& operator >>(istream& is, BigInt& b);
      |            ^~~~~~~
/in/foo.cc:20:1: error: 'ostream' does not name a type
   20 | ostream& operator <<(ostream& os, BigInt& b) {
      | ^~~~~~~
/in/foo.cc:25:1: error: 'istream' does not name a type
   25 | istream& operator >>(istream& is, BigInt& b) {
      | ^~~~~~~