1. A method of operating a digital computer to test an interface between a first program module and a second program module, each of said first and second program modules having associated input variables and an associated cause/effect chart comprising a plurality of cases each having an input condition comprising values of input variables, said method comprising initiating the execution of said first program module, suspending the execution of said first program module at a point where said first program module calls said second program module and verifying that the values of those input variables associated with said second program module that are defined at the time the execution of said first program module is suspended, are consistent with the input condition of at least one of the cases of the cause/effect chart associated with said second program module.