
Abstract interfaces cannot exist on their own. They can be implemented either
as a valuetype or as an interface. Methods can then operate on an abstract
interface without knowing if the parameter is actually a (local) valuetype
or a (potentially remote) object.

This example shows a "Describable" abstract interface, which has the single
method "get_description". This method is then implemented in both the
concrete interface "Account" and the valuetype "Currency" -- both are
describable and can produce a description of themselves as a string.

The "Example" interface can therefore display both Accounts and Currencies,
since they both implement the abstract base.
