We prove that a result of Kirk and Saliga [J. Comput. Appl. Math., 113 (2000), 141-152, Theorem 4.2., p. 149] has been for the first time proved before 25 years in Tasković [Publ. Inst. Math., 41 (1980), 249-258, Theorem 1, p. 250]. But the authors neglected and ignored this historical fact.