(ref.doc)oosc-2/3

2.3 The Open-Closed principle, p 23
A satisfactory modular decomposition technique must satisfy one more
requirement: it should yield modules that are both open and closed.
- A module will be said to be open if it is still available for
  extension.
- A module will be said to be closed if it is available for use by
  other modules.

In classical design approaches, the solution is to close modules when
it appears that they have reached a reasonable level of stability, and
then, when a modification is needed, to reopen them. But if you reopen
a module, you must also reopen all its clients to update them, since
they rely on the old version.