1. A method for specifying a system behavior, comprising: (i) providing a system Graphic User Interface (GUI); the system GUI includes objects; the system GUI is associated with data structure describing at least the GUI objects; the method includes performing steps that include the following steps (ii) and (iii), as many times as required: (ii) playing-in a scenario utilizing the system GUI, the scenario is representative of at least one use case; and (iii) constructing formal system behavior specification that corresponds to the scenario.