diff --git a/btree.h b/btree.h
index 1845bcb..338c237 100644
--- a/btree.h
+++ b/btree.h
@@ -2,6 +2,7 @@
 #define BTREE_H
 
 #include "btypes.h"  // integer definitions
+#include "btree_vfs.h" // bTreeVFS class
 
 enum treeTypes { BT_CUSTOM, BT_PCHAR, BT_STRING, BT_SINGLE, BT_DOUBLE, BT_INT32, BT_INT64 };
 
@@ -55,4 +56,18 @@
 typedef void (*copyKeyProc)(void *, void *);
 typedef int (*keySizeFunc)(void *);
 
+class bTree {
+ protected:
+  compareKeyFunc compareKeys;
+  copyKeyProc    copyKey;
+  keySizeFunc    keySize;
+  bTreeInfo *    info;
+  bTreeVFS *     vfs;
+  bool           memoryTree;
+  bool           treeChanged; 
+ public:
+           bTree(void);
+  virtual ~bTree(void);
+};// bTree
+
 #endif
diff --git a/btree_vfs.h b/btree_vfs.h
new file mode 100644
index 0000000..b51169a
--- /dev/null
+++ b/btree_vfs.h
@@ -0,0 +1,17 @@
+#ifndef BTREE_VFS_H 
+#define BTREE_VFS_H
+
+class bTreeVFS {
+ public:
+                bTreeVFS(void);
+  virtual bool  fClose(void) = 0;
+  virtual bool  fCreate(char * filename) = 0;
+  virtual bool  fExist(char * filename) = 0;
+  virtual bool  fOpen(char * filename) = 0;
+  virtual bool  fRead(char * filename) = 0;
+  virtual bool  fSeek(uInt32 offset) = 0;
+  virtual bool  fWrite(void * buf, uInt32 size) = 0;
+  virtual      ~bTreeVFS(void) = 0;  
+};  // bTreeVFS
+
+#endif
diff --git a/lists.pas b/lists.pas
index 62c917e..033f5f0 100755
--- a/lists.pas
+++ b/lists.pas
@@ -182,7 +182,7 @@
     function    GetLastKey(var searchRec:BTreeSearchRec):boolean;
     procedure   PrintList(const filename:string);
     procedure   PrintWholeTree(const filename:string);
-    destructor  done;
+    destructor  done; virtual;
   end; // bTree
 
 implementation