foo.cc: In function 'int main()':
foo.cc:22:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<b.length();i++)
               ~^~~~~~~~~~~
foo.cc:26:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=0;i<b.length()-1;i++)
               ~^~~~~~~~~~~~~
foo.cc:21:7: warning: unused variable 'l' [-Wunused-variable]
   int l=b.length();
       ^

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