foo.cc: In function 'PolyLinkNode& Mul(PolyLinkNode&, PolyLinkNode&)':
foo.cc:127:9: warning: reference to local variable 'nc' returned [-Wreturn-local-addr]
  127 |  return nc;
      |         ^~
foo.cc:122:15: note: declared here
  122 |  PolyLinkNode nc;
      |               ^~

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