Make VFS wrapper state aware
diff --git a/src/fs-filo-vfs.ads b/src/fs-filo-vfs.ads
index 6115554..2b6dd52 100644
--- a/src/fs-filo-vfs.ads
+++ b/src/fs-filo-vfs.ads
@@ -6,19 +6,53 @@
 
 generic
 
-   with procedure Mount (Success : out Boolean);
+   type T (<>) is private;
 
-   with procedure Open (File_Path : String; Success : out Boolean);
-   with procedure Close with Convention => C;
+   with function Is_Mounted (State : T) return Boolean;
+   with function Is_Open (State : T) return Boolean
+   with
+      Post => (if Is_Open'Result then Is_Mounted (State));
 
-   with procedure Read (Buf : out Buffer_Type; Len : out Natural);
+   Initial : T;
+
+   with procedure Mount
+     (State    : in out T;
+      Part_Len : in     Partition_Length;
+      Success  :    out Boolean)
+   with
+      Pre => not Is_Mounted (State),
+      Post => Success = Is_Mounted (State);
+
+   with procedure Open
+     (State       : in out T;
+      File_Len    :    out File_Length;
+      File_Path   : in     String;
+      Success     : out    Boolean)
+   with
+      Pre => Is_Mounted (State) and not Is_Open (State),
+      Post => Success = Is_Open (State);
+
+   with procedure Close (State : in out T)
+   with
+      Pre => Is_Open (State),
+      Post => Is_Mounted (State);
+
+   with procedure Read
+     (State    : in out T;
+      File_Len : in     File_Length;
+      File_Pos : in out File_Offset;
+      Buf      :    out Buffer_Type;
+      Len      :    out Natural)
+   with
+      Pre => Is_Open (State),
+      Post => Is_Open (State);
 
 package FS.FILO.VFS is
 
    function C_Mount return int;
 
    function C_Open (File_Path : Strings.chars_ptr) return int;
-   procedure C_Close renames Close;
+   procedure C_Close;
    
    function C_Read (Buf : System.Address; Len : int) return int;