Consider some items in the the list in the article on "modularity":

biology
construction
ecology
cognitive science
industrial design
manufacturing
modular programming
contemporary art and architecture
software design

The editors try this out as a first definition: "Product systems are deemed “modular”, for example, when they can be decomposed into a number of components that may be mixed and matched in a variety of configurations."1 2

In mathematics, the term can be used in several different ways. A module over a ring is a generalization of the notion of vector space over a field. Modular lattices are partially ordered sets which generalize the order structure of submodules. The modularity theorem (formerly the Taniyama–Shimura conjecture) establishes a connection between elliptic curves and modular forms.

Black box. Flow charts. Stepwise refinement.

QUESTION: how does Wirth's (1971) notion of stepwise refinement relate to hierarchical planning?