diff --git a/ExtraTypes.cpp b/ExtraTypes.cpp
index e5897b585985943002c64b57c6e4e5ad301011f6..eb054d612a24231318d4c16d3a1d9d512cc1ab07 100644
--- a/ExtraTypes.cpp
+++ b/ExtraTypes.cpp
@@ -97,3 +97,24 @@ bool Edge::operator==(const Edge& right_edge) const {
 bool Edge::operator<(const Edge& right_edge) const {
     return c_id < right_edge.c_id;
 }
+
+template < typename _T >
+inline void hashCombine ( std::size_t & o_seed , const _T & p_val)
+{
+ o_seed ^= std :: hash <_T >() (p_val) + 0x9e3779b9 + (o_seed << 6) + (o_seed >> 2) ;
+}
+
+size_t hash_u_set<s>::operator()(const s &x) const{
+    std :: size_t seed = 0;
+    int a[] = {x.f_id, x.l_id, x.c_id};
+    if(a[0] > a[1])
+        a[0].swap(a[1]);
+    if(a[1] > a[2])
+        a[1].swap(a[2]);
+    if(a[0] > a[1])
+        a[0].swap(a[1]);
+    hashCombine(seed, a[0]);
+    hashCombine(seed, a[1]);
+    hashCombine(seed, a[2]);
+    return seed;
+}
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