oughSet</h2>

<p>
Units, cities and foreign cities are changing sets of objects. These items might not only accumulate (as this is the case for models) but also disappear. To manage them in lists could easily lead to problems, because indices in a list would invite the programmer to use them for identification purpose, which would not work in the long term. So these objects are not managed using lists but in a collection class called ToughSet. That one only provides an enumeration (to use with foreach) but 