@roulis - I've said before, the computer code is not manually keyed in in that manner. Code blocks are tested in situ against the formalised function, then assigned to an element in a graphical development environment, which then generates the logic path from those individually-tested blocks of code. Each subsystem is then itself tested against the required behaviour pattern, and as the subsystems are linked together to form an overall program "module", that module and its functions are tested individually and in conjunction with others.
These tests are run *every single time* a change is made, and the results of the tests plotted against a regression graph. Any change in that graph is immediately apparent, and will point to exactly where the program deviates from the specified behaviour.