As the detection of errors in data is achieved through duplicating all variables and adding consistency checks after every read operation, special considerations have to be applied according to the procedure interfaces. Parameters passed to procedures, as well as return values, are considered to be variables. Hence, every procedure parameter is duplicated, as well as the return values. A procedure is still called only once, but it returns two results, which must hold the same value. The source listing to the right shows a sample implementation of function parameter duplication.