we demonstrate a formal technique used for evolution
of hierarchically decomposed systems.

The systems and components are specified through their
input/output behavior based on Focus' streams.

Internal evolution shall respect the promised external behavior without
vialoating it.

The evolution calculus provides simple transformations that 
either ensure correctnes of changes automatically or
produce additional (behavioral) proof conditions.

Message is:
Evolution can be assisted using an appropriate calculus of
small, composable steps.