erasing objects from dynamic windows

["C. Ann Dent" <Ann@drea-phoenix.arpa>]

I want to display an object upon a dynamic window such that its display
remains visible whenever the viewport is scrolled to show some of the
area where the object was displayed.  However, I would like to be able
to erase the display of that object at a later time (such that its
display will no longer visible even if the viewport is scrolled to show
the location where it was originally displayed).

This seems like such a trivial task that I suspect the solution must be
obvious.  Any suggestions or pointers? 

	Ann

P.S.  By the way, the size of the display of object may be larger than
the size of the viewport to the dynamic window.  Also, the location of
its display may lie totally outside the boundary of the viewport of the
window when the object is originally displayed. 
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