Set window property prop of win to val. prop should be a symbol. val may be any Scheme object. This name/value pair will be associated with the window, and may be retrieved with window-property. Passing #f as the value will delete the property instead. Soon, some properties will have magical meanings, altering particular fields in the window structure. Also, a window-property-change-hook mechanism will soon be implemented for notification of all window property changes. This is not yet done. The window property primitives should be considered in flux.