/in/foo.cc: In function 'int main()':
/in/foo.cc:42:27: warning: 'a.PolyLink::tail' may be used uninitialized in this function [-Wmaybe-uninitialized]
                 tail->next=p;
                 ~~~~~~~~~~^~
/in/foo.cc:59:14: note: 'a.PolyLink::tail' was declared here
     PolyLink a(n);
              ^