Add concept Hashable

STL_Extension/doc/STL_Extension/Concepts/Hashable.h

@@ -0,0 +1,23 @@
+
+/*!
+\ingroup PkgStlExtensionConcepts
+\cgalConcept
+
+A type `Key` is a model of the concept `Hashable` if the
+specializations `boost::hash<Key>` and `std::hash<Key>` exist. 
+
+\cgalHasModel All handles and indices of \cgal data structures.
+
+
+\sa `CGAL::Unique_hash_map<Key,Data,Hash>` 
+\sa `std::unordered_set`
+\sa `std::unordered_map`
+\sa `boost::unordered_set`
+\sa `boost::unordered_map`
+
+*/
+
+class Hashable {
+
+};
+