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Problem with draw-image and HD 1.2

I created a subclass of <image> (called <windows-bitmap>) and added
methods to image-width, image-height, etc for the image protocols so
it would work with the various DUIM routines. When it came to doing
draw-image I tried specialising the first parameter on <medium>:

  define method draw-image( m :: <medium>, 
    image :: <windows-bitmap>, x, y) => (record) /* etc */ end;

This compiled, but when run I got an error saying that the above
method "cannot be added to the generic function draw-image because it
is blocked by the sealed domain over <win32-medium>, <image>,
<object>, <object> in <library>. Oops!"

In the end I had to make it specialised over <drawing-pane> to work:

  define method draw-image( pane :: <drawing-pane>, 
    image :: <windows-bitmap>, x, y) => (record) /* etc */ end;

And make sure I called draw-image with a <drawing-pane> instead of a
<medium>. Is there another workaround for this? What sealing
declaration is likely to be causing the problem? I don't know much
about the intricacies of sealing unfortunately. I have no sealing
declarations in my own code apart from what occurs by default (not
declaring classes open, etc).

The working version of the project (bitmap-example) is available at
http://www.double.co.nz/dylan, where is it specialised over
<drawing-pane>. Changing this to <medium> will demonstrate the
problem. 

As an aside, the project mentioned above shows loading bitmaps from
files using the Windows API and displaying it in a drawing-pane. I
tried using <win32-bitmap> instead of creating my own <windows-bitmap>
class but <win32-bitmap>'s sometimes did not display correctly when
using draw-image in HD1.2. 

Chris.
-- 
http://www.double.co.nz/dylan
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