The original series of steps for our solution, called
the Top Level Solution were numbered in
sequence. (1 to 3 in our example)
In our second stage, the “sub-steps” were numbered in such a way that their position in the algorithm is
clear. (As they were sub-steps of step 2, they
were numbered 2.1, 2.2 etc.)
Further levels of decomposition are possible, for
example if we split stage 2.1 up, they
would be numbered 2.1.1, 2.1.2 etc.