Modular = parts are relatively independent at a given level. Weak coupling between them. You can change
one without breaking the others:

Near-decomposability

A system is nearly decomposable when interactions within subsystems are much stronger than interactions between subsystems.


(a) Short-run behavior of each subsystem is approximately independent of other subsystems.
(b) Long-run behavior depends on other subsystems only in an aggregate way.

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Hierarchical modularity of nested bow-ties in metabolic networks
Developmental encodings promote the emergence of hierarchical modularity across very different mechanisms