/in/foo.cc: In function 'int main()':
/in/foo.cc:9:9: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(head<s.size())
     ~~~~^~~~~~~~~
/in/foo.cc:11:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   if(tail<s.size()) s.erase(head,tail);
      ~~~~^~~~~~~~~