1. A method of verifying a computer source code having a series of program statements, including control statements which provide for branching to alternative statements of the code, said method comprising: instrumenting the source code to generate when the code is executed an indication for each block of code that the statements of that block have been executed, a block of code being the program statements between control statements; selecting test cases, with each test case comprising input values for variables in the source code, said test cases being selected such that each block of code is executed; generating a test driver routine to implement the test cases; compiling and linking the test driver routine with the instrumented source code; executing the compiled/linked code to implement the test cases, and generating an output with said indication of each block of code which has been executed by the test cases.