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              The :selection-request event is reported to the owner 
              of a selection when a client calls 
              convert-selection. This event requests the selection 
              owner to convert the current value of a selection into a 
              specified form and to return it to the requestor. A client can 
              neither select :selection-request events nor avoid receiving them.
             
              The selection owner should respond to a :selection-request event by performing the 
              following steps:
             
              
                Convert the current selection value to the target
                 type.
              
                Store the converted selection value in the property. 
                If property is nil, then the owner should 
                choose the property.
              
                Call 
                send-event to send a :selection-notify event to the requestor 
                containing the property with the converted value. If 
                the selection could not be converted to the target
                 type, then a nil property should be sent. 
                The :selection, :target, and :time 
                arguments to 
                send-event should be the same as those received in 
                the :selection-request event. The event-mask argument 
                to send-event
                 should be nil; that is, the :selection-notify event should be sent to client 
                that created the requestor.
             
              NOTE: Standard conventions for inter-client 
              communication require the following additional steps in 
              processing a :selection-request event:
             
              
                The property used to store the selection value must belong to 
                the requestor.
              
                If the property is nil, the target type atom 
                should be used as the property name.
              
                If the window did not actually own the selection at the given 
                time, the request should be refused, just as if it could not 
                be converted to the target type.
             
              
                window, event-window
              
                Type window. 
                The selection owner receiving the event.
              
                requestor
              
                Type window. 
                The window requesting the converted selection.
              
                selection
              
                Type keyword. The selection to be converted.
              
                target
              
                Type keyword. An xatom specifying the type of the converted 
                selection value.
              
                property
              
                Type (or null keyword). A requestor window property.
              
                time
              
                Type timestamp
                . The timestamp sent in the client 
                convert-selection request.
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