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diff --git a/Source/cmDefinitions.h b/Source/cmDefinitions.h
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+/*============================================================================
+  CMake - Cross Platform Makefile Generator
+  Copyright 2000-2009 Kitware, Inc., Insight Software Consortium
+
+  Distributed under the OSI-approved BSD License (the "License");
+  see accompanying file Copyright.txt for details.
+
+  This software is distributed WITHOUT ANY WARRANTY; without even the
+  implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+  See the License for more information.
+============================================================================*/
+#ifndef cmDefinitions_h
+#define cmDefinitions_h
+
+#include "cmStandardIncludes.h"
+
+/** \class cmDefinitions
+ * \brief Store a scope of variable definitions for CMake language.
+ *
+ * This stores the state of variable definitions (set or unset) for
+ * one scope.  Sets are always local.  Gets search parent scopes
+ * transitively and save results locally.
+ */
+class cmDefinitions
+{
+public:
+  /** Construct with the given parent scope.  */
+  cmDefinitions(cmDefinitions* parent = 0);
+
+  /** Reset object as if newly constructed.  */
+  void Reset(cmDefinitions* parent = 0);
+
+  /** Returns the parent scope, if any.  */
+  cmDefinitions* GetParent() const { return this->Up; }
+
+  /** Get the value associated with a key; null if none.
+      Store the result locally if it came from a parent.  */
+  const char* Get(const char* key);
+
+  /** Set (or unset if null) a value associated with a key.  */
+  const char* Set(const char* key, const char* value);
+
+  /** Get the set of all local keys.  */
+  std::set<cmStdString> LocalKeys() const;
+
+  /** Compute the closure of all defined keys with values.
+      This flattens the scope.  The result has no parent.  */
+  cmDefinitions Closure() const;
+
+  /** Compute the set of all defined keys.  */
+  std::set<cmStdString> ClosureKeys() const;
+
+private:
+  // String with existence boolean.
+  struct Def: public cmStdString
+  {
+    Def(): cmStdString(), Exists(false) {}
+    Def(const char* v): cmStdString(v?v:""), Exists(v?true:false) {}
+    Def(Def const& d): cmStdString(d), Exists(d.Exists) {}
+    bool Exists;
+  };
+  static Def NoDef;
+
+  // Parent scope, if any.
+  cmDefinitions* Up;
+
+  // Local definitions, set or unset.
+  typedef std::map<cmStdString, Def> MapType;
+  MapType Map;
+
+  // Internal query and update methods.
+  Def const& GetInternal(const char* key);
+  Def const& SetInternal(const char* key, Def const& def);
+
+  // Implementation of Closure() method.
+  struct ClosureTag {};
+  cmDefinitions(ClosureTag const&, cmDefinitions const* root);
+  void ClosureImpl(std::set<cmStdString>& undefined,
+                   cmDefinitions const* defs);
+
+  // Implementation of ClosureKeys() method.
+  void ClosureKeys(std::set<cmStdString>& defined,
+                   std::set<cmStdString>& undefined) const;
+};
+
+#endif