Add skeleton for theory of bags (multisets) (#5100)

This PR adds initial headers and mostly empty source files for the theory of bags (multisets).
It acts as a basis for future commits.
It includes straightforward implementation for typing rules an enumerator for bags.

1 files changed, 63 insertions, 0 deletions

diff --git a/src/expr/emptybag.h b/src/expr/emptybag.h
new file mode 100644
index 000000000..a16c12d86
--- /dev/null
+++ b/src/expr/emptybag.h
@@ -0,0 +1,63 @@
+/*********************                                                        */
+/*! \file emptybag.h
+ ** \verbatim
+ ** Top contributors (to current version):
+ **   Mudathir Mohamed
+ ** This file is part of the CVC4 project.
+ ** Copyright (c) 2009-2020 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 class for empty bags
+ **/
+
+#include "cvc4_public.h"
+
+#ifndef CVC4__EMPTY_BAG_H
+#define CVC4__EMPTY_BAG_H
+
+#include <iosfwd>
+#include <memory>
+
+namespace CVC4 {
+
+class TypeNode;
+
+class CVC4_PUBLIC EmptyBag
+{
+ public:
+  /**
+   * Constructs an emptybag of the specified type. Note that the argument
+   * is the type of the bag itself, NOT the type of the elements.
+   */
+  EmptyBag(const TypeNode& bagType);
+  ~EmptyBag();
+  EmptyBag(const EmptyBag& other);
+  EmptyBag& operator=(const EmptyBag& other);
+
+  const TypeNode& getType() const;
+  bool operator==(const EmptyBag& es) const;
+  bool operator!=(const EmptyBag& es) const;
+  bool operator<(const EmptyBag& es) const;
+  bool operator<=(const EmptyBag& es) const;
+  bool operator>(const EmptyBag& es) const;
+  bool operator>=(const EmptyBag& es) const;
+
+ private:
+  EmptyBag();
+
+  /** the type of the empty bag itself (not the type of the elements)*/
+  std::unique_ptr<TypeNode> d_type;
+}; /* class EmptyBag */
+
+std::ostream& operator<<(std::ostream& out, const EmptyBag& es) CVC4_PUBLIC;
+
+struct CVC4_PUBLIC EmptyBagHashFunction
+{
+  size_t operator()(const EmptyBag& es) const;
+}; /* struct EmptyBagHashFunction */
+
+}  // namespace CVC4
+
+#endif /* CVC4__EMPTY_BAG_H */