foo.cc: In function 'int main()':
foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=0;i<a.size();i++)
              ~^~~~~~~~~
foo.cc:18:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i=1;i<=a.size();i++) 
              ~^~~~~~~~~~

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