/in/foo.cc: In function 'int main()':
/in/foo.cc:2:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 #define fo(i,l,r) for(int i=l;i<=r;i++)
                                 
/in/foo.cc:15:5:
  fo(i,0,s.size()-1){
     ~~~~~~~~~~~~~~              
/in/foo.cc:15:2: note: in expansion of macro 'fo'
  fo(i,0,s.size()-1){
  ^~