foo.cpp:6:19: error: 'null_mapped_type' was not declared in this scope
 typedef tree<int, null_mapped_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> Set;
                   ^
foo.cpp:6:94: error: template argument 2 is invalid
 typedef tree<int, null_mapped_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> Set;
                                                                                              ^
foo.cpp:6:99: error: invalid type in declaration before ';' token
 typedef tree<int, null_mapped_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> Set;
                                                                                                   ^
foo.cpp: In function 'int main()':
foo.cpp:15:5: error: request for member 'insert' in 't', which is of non-class type 'Set {aka int}'
   t.insert(a[i]);
     ^
foo.cpp:23:14: error: request for member 'order_of_key' in 't', which is of non-class type 'Set {aka int}'
    int o = t.order_of_key(i);
              ^
foo.cpp:24:22: error: request for member 'find_by_order' in 't', which is of non-class type 'Set {aka int}'
    printf("%d\n", *t.find_by_order(o+k-1));
                      ^
foo.cpp:28:6: error: request for member 'erase' in 't', which is of non-class type 'Set {aka int}'
    t.erase(t.find(a[i]));
      ^
foo.cpp:28:14: error: request for member 'find' in 't', which is of non-class type 'Set {aka int}'
    t.erase(t.find(a[i]));
              ^
foo.cpp:30:6: error: request for member 'insert' in 't', which is of non-class type 'Set {aka int}'
    t.insert(j);
      ^