a program is constituted by some code intended to determine a specific behav- ior inside the computer. Such behavior is specified by a program specifica- ttiioonn..  a software system is constituted by a program intended to determine a specific external behavior of the machine (at its interface with the environment). Such eexxtteerrnnaall bbeehhaavviioorr iiss ssppeecciiffiieedd bbyy aa ssooffttwwaarree ssyysstteemm ssppeecciiffiiccaattiioonn..  a software product is constituted by a software system designed to determine specific effects in the environment as a result of the machine behavior, under given domain assumptions. Such effects are specified by the high level re- quirements.