/in/foo.cc: In function 'int main()':
/in/foo.cc:10:46: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 #define rep(i,a,b) for(register int i = a; i < b; ++ i)
                                               
/in/foo.cc:37:6:
  rep(i,0,used.size()) {
      ~~~~~~~~~~~~~~~                          
/in/foo.cc:37:2: note: in expansion of macro 'rep'
  rep(i,0,used.size()) {
  ^~~