foo.cc: In function 'std::__cxx11::string left(std::__cxx11::string, int)':
foo.cc:18:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = len; i < x.length(); i++)
                    ~~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:31:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < x.length(); i++) {
                  ~~^~~~~~~~~~~~
foo.cc:43:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < x.length(); i++)
                  ~~^~~~~~~~~~~~

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