In file included from /usr/include/c++/9/map:60,
from /usr/include/x86_64-linux-gnu/c++/9/bits/stdc++.h:81,
from golf.cpp:8:
/usr/include/c++/9/bits/stl_tree.h: In instantiation of 'static const _Key& std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_S_key(std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Const_Link_type) [with _Key = std::pair<int, int>; _Val = std::pair<int, int>; _KeyOfValue = std::_Identity<std::pair<int, int> >; _Compare = cells_order; _Alloc = std::allocator<std::pair<int, int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_Const_Link_type = const std::_Rb_tree_node<std::pair<int, int> >*]':
/usr/include/c++/9/bits/stl_tree.h:2095:47: required from 'std::pair<std::_Rb_tree_node_base*, std::_Rb_tree_node_base*> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_get_insert_unique_pos(const key_type&) [with _Key = std::pair<int, int>; _Val = std::pair<int, int>; _KeyOfValue = std::_Identity<std::pair<int, int> >; _Compare = cells_order; _Alloc = std::allocator<std::pair<int, int> >; std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::key_type = std::pair<int, int>]'
/usr/include/c++/9/bits/stl_tree.h:2148:4: required from 'std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(_Arg&&) [with _Arg = std::pair<int, int>; _Key = std::pair<int, int>; _Val = std::pair<int, int>; _KeyOfValue = std::_Identity<std::pair<int, int> >; _Compare = cells_order; _Alloc = std::allocator<std::pair<int, int> >]'
/usr/include/c++/9/bits/stl_set.h:520:48: required from 'std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(std::set<_Key, _Compare, _Alloc>::value_type&&) [with _Key = std::pair<int, int>; _Compare = cells_order; _Alloc = std::allocator<std::pair<int, int> >; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree_const_iterator<std::pair<int, int> >; std::set<_Key, _Compare, _Alloc>::value_type = std::pair<int, int>]'
golf.cpp:54:48: required from here
/usr/include/c++/9/bits/stl_tree.h:780:8: error: static assertion failed: comparison object must be invocable as const
780 | is_invocable_v<const _Compare&, const _Key&, const _Key&>,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~