foo.cc: In member function 'BigInt BigInt::Mul(int)':
foo.cc:66:19: error: cannot bind non-const lvalue reference of type 'BigInt&' to an rvalue of type 'BigInt'
   66 |         return Mul(tmp);
      |                ~~~^~~~~
foo.cc:10:20: note:   initializing argument 1 of 'BigInt::BigInt(BigInt&)'
   10 |     BigInt(BigInt &source) {
      |            ~~~~~~~~^~~~~~
foo.cc: In function 'int main()':
foo.cc:85:22: error: cannot bind non-const lvalue reference of type 'BigInt&' to an rvalue of type 'BigInt'
   85 |     BigInt p1 = Power(x1, n1);
      |                 ~~~~~^~~~~~~~
foo.cc:10:20: note:   initializing argument 1 of 'BigInt::BigInt(BigInt&)'
   10 |     BigInt(BigInt &source) {
      |            ~~~~~~~~^~~~~~
foo.cc:86:22: error: cannot bind non-const lvalue reference of type 'BigInt&' to an rvalue of type 'BigInt'
   86 |     BigInt p2 = Power(x2, n2);
      |                 ~~~~~^~~~~~~~
foo.cc:10:20: note:   initializing argument 1 of 'BigInt::BigInt(BigInt&)'
   10 |     BigInt(BigInt &source) {
      |            ~~~~~~~~^~~~~~

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