/in/foo.cc: In function 'void add(int)':
/in/foo.cc:9:5: error: reference to 'next' is ambiguous
     next[tot]=head[a%mod];
     ^~~~
/in/foo.cc:5:29: note: candidates are: int next [2333343]
 int head[mod+10],to[mod+10],next[mod+10],tot;
                             ^~~~
In file included from /usr/include/c++/7/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/7/bits/stl_iterator_base_funcs.h:208:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:26:35: error: reference to 'next' is ambiguous
         for(int j=head[x%mod];j;j=next[j])
                                   ^~~~
/in/foo.cc:5:29: note: candidates are: int next [2333343]
 int head[mod+10],to[mod+10],next[mod+10],tot;
                             ^~~~
In file included from /usr/include/c++/7/bits/stl_algobase.h:66:0,
                 from /usr/include/c++/7/bits/char_traits.h:39,
                 from /usr/include/c++/7/ios:40,
                 from /usr/include/c++/7/ostream:38,
                 from /usr/include/c++/7/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/7/bits/stl_iterator_base_funcs.h:208:5: note:                 template<class _ForwardIterator> _ForwardIterator std::next(_ForwardIterator, typename std::iterator_traits<_Iter>::difference_type)
     next(_ForwardIterator __x, typename
     ^~~~