#include <bitcoin/bitcoin.hpp>

  int main()
  {
      libbitcoin::wallet::hd_lineage a, b;
      a < b;
      return 0;
  }

$ g++ main.cpp $(pkg-config --cflags --libs libbitcoin) -std=c++11
/tmp/ccd5MFwR.o: In function `main':
main.cpp:(.text+0x26): undefined reference to
`libbitcoin::wallet::hd_lineage::operator<(libbitcoin::wallet::hd_lineage
const&) const'
collect2: error: ld returned 1 exit status

I looked at the source code and everything seems fine (operator is
implemented fine).

This is only with the < operator. Both == and != work fine.