Supposing a program composed of parts A B C and D and a requirement for program of parts A B C and E.

The effort required to construct this program should be proportional to the size of E. The extent to which this is true is the extent to which one achieves the central thesis of Functional Programming.

Identifying independent program parts requires very rigorous cognitive discipline and correct concept formation. This can be very (very) difficult after exposure to sloppy thinking habits.