procedure delete ( var pq : tree );
	begin
	if pq = nil then Error {*** deletion on an empty queue ***}
	else if pq^.left = nil then pq := nil
	else if pq^.left^.left = nil then begin
		pq^.left := pq^.right;
		pq^.right := nil
		end
	else	delete( pq^.left )
	end;