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+/*********************                                                        */
+/*! \file skolem_cache.h
+ ** \verbatim
+ ** Top contributors (to current version):
+ **   Andrew Reynolds
+ ** This file is part of the CVC4 project.
+ ** Copyright (c) 2009-2019 by the authors listed in the file AUTHORS
+ ** in the top-level source directory) and their institutional affiliations.
+ ** All rights reserved.  See the file COPYING in the top-level source
+ ** directory for licensing information.\endverbatim
+ **
+ ** \brief A cache of skolems for theory of sets.
+ **/
+
+#include "cvc4_private.h"
+
+#ifndef CVC4__THEORY__SETS__SKOLEM_CACHE_H
+#define CVC4__THEORY__SETS__SKOLEM_CACHE_H
+
+#include <map>
+#include <unordered_set>
+
+#include "expr/node.h"
+
+namespace CVC4 {
+namespace theory {
+namespace sets {
+
+/**
+ * A cache of all set skolems generated by the TheorySets class. This
+ * cache is used to ensure that duplicate skolems are not generated when
+ * possible, and helps identify what skolems were allocated in the current run.
+ */
+class SkolemCache
+{
+ public:
+  SkolemCache();
+  /** Identifiers for skolem types
+   *
+   * The comments below document the properties of each skolem introduced by
+   * inference in the sets solver, where by skolem we mean the fresh
+   * set variable that witnesses each of "exists k".
+   */
+  enum SkolemId
+  {
+    // (a,b) in join(A,B) => exists k. (a,k) in A ^ (k,b) in B
+    // This is cached by the nodes corresponding to (a,b) and join(A,B).
+    SK_JOIN,
+  };
+
+  /**
+   * Makes a skolem of type tn that is cached based on the key (a,b,id).
+   * Argument c is the variable name of the skolem.
+   */
+  Node mkTypedSkolemCached(
+      TypeNode tn, Node a, Node b, SkolemId id, const char* c);
+  /** Same as above, but without caching. */
+  Node mkTypedSkolem(TypeNode tn, const char* c);
+  /** Returns true if n is a skolem allocated by this class */
+  bool isSkolem(Node n) const;
+
+ private:
+  /** map from node pairs and identifiers to skolems */
+  std::map<Node, std::map<Node, std::map<SkolemId, Node> > > d_skolemCache;
+  /** the set of all skolems we have generated */
+  std::unordered_set<Node, NodeHashFunction> d_allSkolems;
+};
+
+}  // namespace sets
+}  // namespace theory
+}  // namespace CVC4
+
+#endif /* CVC4__THEORY__STRINGS__SKOLEM_CACHE_H */