/in/foo.cc: In function 'int main()':
/in/foo.cc:8:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s1.size();i++)
              ~^~~~~~~~~~
/in/foo.cc:12:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<s2.size();i++)
              ~^~~~~~~~~~
/in/foo.cc:18:29: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while((pos=s2.find(s1,pos))!=string::npos)
        ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~