I've just learned about ordered set. I tried to use these templates

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> ordered_set;
typedef tree<int, null_type, less_equal<int>, rb_tree_tag, tree_order_statistics_node_update> multi_ordered_set;

But I'm using Macbook, so I couldn't compile it. How can I fix that? Thanks for your help.