A module maps function to structure.
Notes from [SWC98]:
Dominant flow heuristic. A single flow that passes all the way through the system. The functions through which the flow passes is a module. Other flows may cross the dominant one; these represent interfaces with the module.
Branching flow heuristic. Given a flow that splits into branches, each branch can be a module. This means modules, like systems, are recursive. This in essence is like finding the subtrees of a tree graph.
Conversion-Transmission heuristic. A function chain that either (a) converts a flow, (b) converts a flow and then transports it, or © converts a flow then performs other functions and finally transports the flow, can be made a module.
TODO It would be nice to get the database of product decompositions that were used in this work. Apparently at UT Austin, referenced by other work I need to look up.