/in/foo.cc:458:20: warning: 'std::string std::to_string(int)' defined but not used [-Wunused-function]
  458 | static std::string to_string(const int value)
      |                    ^~~~~~~~~
/usr/bin/ld: /tmp/cctZ8xMT.o: in function `std::divide(std::Bigint, std::Bigint, std::vector<std::Bigint, std::allocator<std::Bigint> >&)':
foo.cc:(.text+0x5a41): undefined reference to `std::sub_number(std::Bigint&, std::Bigint&)'
collect2: error: ld returned 1 exit status