  Systems of systems differ from traditional systems in that they are open at
the top, open at the bottom, and continually (but slowly) evolving. "Open at
the top" means that there is no pre-defined top level application. New
applications may be created at any time. "Open at the bottom" means that the
system primitives are defined functionally rather than concretely. This allows
the implementation of these primitives to be modified as technology changes.
"Continually (but slowly) evolving" means that the system's functionality is
stable enough to be useful but is understood to be subject to modification.
Systems with these properties tend to be environments within which other
systems operate--and hence are systems of systems. It is also important to
understand the larger environment within which a system of systems exists.
