/in/foo.cc: In function 'void dfs(int)':
/in/foo.cc:5:11: error: 'first' was not declared in this scope
 #define x first
           ^
/in/foo.cc:15:39: note: in definition of macro 'F'
 #define F(i,start,end) for(R int (i)=(start);(i)<=(end);i++)
                                       ^~~~~
/in/foo.cc:100:8: note: in expansion of macro 'x'
  F(i,c[x],m)
        ^