/in/foo.cc: In function 'int main()':
/in/foo.cc:22:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < va.size(); i++) {
                  ~~^~~~~~~~~~~
/in/foo.cc:25:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < vb.size(); i++) {
                  ~~^~~~~~~~~~~
/in/foo.cc:29:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < vc.size(); i++) {
                  ~~^~~~~~~~~~~