Masking and transparency

I'm trying to do what should be fairly simple: mask parts of an image
(using a greyscale mask - some parts could be masked out, others would
have varying levels of transparency based on the darkness of the mask)
and compose it on a background.  The code below builds a mask and uses
CopyCompositeOp to set the alpha channel of the top layer, but the
resulting image is just the mask composed on the blue background rather
than a masked copy of the top image.

What's the proper series of function calls to get this done properly?

thx --

    ,----
    | #include <wand/magick_wand.h>
    | 
    | int
    | main (void)
    | {
    |   MagickWand *background;
    |   PixelWand *pw;
    |   MagickWand *img;
    |   MagickWand *mask;
    |   DrawingWand *dw;
    |   MagickBooleanType rc;
    | 
    |   MagickWandGenesis ();
    | 
    |   background = NewMagickWand ();
    |   pw = NewPixelWand ();
    | 
    |   rc = PixelSetColor (pw, "blue");
    |   rc = MagickNewImage (background, 640, 800, pw);
    | 
    |   img = NewMagickWand ();
    | 
    |   rc = MagickReadImage (img, "test-image.png");
    | 
    |   mask = NewMagickWand ();
    | 
    |   rc = PixelSetColor (pw, "gray");
    |   rc = MagickNewImage (mask, MagickGetImageWidth (img), MagickGetImageHeight (img), pw);
    | 
    |   dw = NewDrawingWand ();
    | 
    |   rc = PixelSetColor (pw, "white");
    |   DrawSetFillColor (dw, pw);
    |   DrawRectangle (dw, 10.0, 10.0, 90.0, 90.0);
    |   rc = MagickDrawImage (mask, dw);
    | 
    |   DestroyDrawingWand (dw);
    | 
    |   rc = MagickCompositeImageChannel (img, AlphaChannel, mask, CopyCompositeOp, 0, 0);
    | 
    |   DestroyMagickWand (mask);
    | 
    |   rc = MagickCompositeImage (background, img, OverCompositeOp, 40, 40);
    | 
    |   DestroyMagickWand (img);
    | 
    |   DestroyPixelWand (pw);
    | 
    |   MagickDisplayImage (background,":0");
    | 
    |   DestroyMagickWand (background);
    | 
    |   MagickWandTerminus();
    | 
    |   return 0;
    | }
    `----

-- 
Michael J. Barillier   ///   http://www.blackwolfinfosys.net/~blackwolf/
_O_|  "... Ideas are bulletproof."
__O|                     -- V
OOO|