Mo, they don't. They are merely a prerequisite. It must be reasonably possible to reimplement the module in question and it must be possible to instantiate that

Mo, they don't. They are merely a prerequisite. It must be reasonably possible to reimplement the module in question and it must be possible to instantiate that alternative implementation instead of the original one. A counter-example is to have all lodules linked into a single static binary. You may have a beautiful separation berween modules in there, but the static linking makes them an immutable bundle.