/in/foo.cc: In function 'int main()':
/in/foo.cc:34:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<Violet>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   34 |   for(int i=0;i<M.size()-1;++i)
      |               ~^~~~~~~~~~~
/in/foo.cc:37:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<Violet>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   37 |    for(int j=i+1;j<M.size();)
      |                  ~^~~~~~~~~