/in/foo.cc: In function 'void work(int, int)':
/in/foo.cc:16:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 0; i < e[x].size(); i++){
                  ~~^~~~~~~~~~~~~
/in/foo.cc: In function 'void p_work()':
/in/foo.cc:40:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i = 0; i < e[x].size(); i++){
                  ~~^~~~~~~~~~~~~
/in/foo.cc:44:5: error: 'pre' was not declared in this scope
     pre[y] = x;  
     ^~~