WIP just need to identify nodes on begin -> end path

Internal.h

@@ -12,6 +12,7 @@
 #include <set>
 #include <span>
 #include <string>
+#include <unordered_set>
 #include <utility>
 #include <vector>
 
@@ -183,6 +184,12 @@ template <class T> using Set = std::set<T, std::less<T>, ArenaAlloc<T>>;
 template <class T> auto set(Arena &arena) {
   return Set<T>(ArenaAlloc<T>(&arena));
 }
+template <class T>
+using HashSet =
+    std::unordered_set<T, std::hash<T>, std::equal_to<T>, ArenaAlloc<T>>;
+template <class T> auto hashSet(Arena &arena) {
+  return HashSet<T>(ArenaAlloc<T>(&arena));
+}
 
 template <class T, class U>
 bool operator==(const ArenaAlloc<T> &lhs, const ArenaAlloc<U> &rhs) {