foo.cc:63:45: error: empty character constant
   63 |                                         put('');
      |                                             ^~
foo.cc: In function ‘bool cmp(Node, Node)’:
foo.cc:10:41: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   10 |         return x.num<y.num||x.num==y.num&&x.id<y.id;
      |                             ~~~~~~~~~~~~^~~~~~~~~~~
foo.cc: In function ‘int main()’:
foo.cc:63:41: error: ‘put’ was not declared in this scope; did you mean ‘putw’?
   63 |                                         put('');
      |                                         ^~~
      |                                         putw