This paper presents a systematic approach, explicitly based on task analysis, to directing the sequence design of operations in user interface. The differences among people in the task knowledge should be reflected in optimizing the operation sequences. The suggested method first describes the task knowledge and derives from it the desired features of logical user-device interaction. Such features of interface to profit from any presumed task knowledge are closely associated with the ways of abstraction. Two types of abstract notions, abstract operations and state closures, are defined and their correspondence to operational features are demonstrated.