2 Why We Need Model-Based Testing

162 Modeling Systems with Structured State
10.3.2 Maps
Union, intersection, and difference
Any two sets of the same type can be combined by union, intersection, and difference.
Union produces the set with all of the elements. Intersection is the set of elements
shared by the two sets. Difference is the set of elements from the first set that are
not found in the second set.
Set s1 = new Set(1, 2, 3, 4, 5, 6);
Set s2 = new Set(1, 3, 5, 7, 9, 11);
Set s3 = s1.Union(s2);
Set s4 = s1.Intersect(s2);
Set s5 = s1.Difference(s2);
Assert.AreEqual(s3, new Set(1, 2, 3, 4, 5, 6, 7, 9, 11));
Assert.AreEqual(s4, new Set(1, 3, 5));
Assert.AreEqual(s5, new Set(2, 4, 6));
You can write union as +, intersection as *, and difference as -.
Set s1 = new Set(1, 2, 3, 4, 5, 6);
Set s2 = new Set(1, 3, 5, 7, 9, 11);
Set s3 = s1 + s2;
Set s4 = s1 * s2;
Set s5 = s1 - s2;
Assert.AreEqual(s3, new Set(11, 3, 2, 9, 6, 5, 7, 4, 1));
Assert.AreEqual(s4, new Set(3, 1, 5));
Assert.AreEqual(s5, new Set(6, 4, 2));
Maps associate unique keys with values. This is similar to the .NET Dictionary type,
except that a map is an immutable value. Operations to add and remove elements
result in new maps instead of making changes to the map given as the argument.
Another difference between maps and the .NET dictionary type is structural
equality. Two maps of the same type are equal if they contain the same key/value
pairs. Dictionaries in .NET use reference equality instead of structural equality.
Also, unlike the .NET dictionary type, the value null may be used as a key in a
map.
Appendix A.2.3 provides a list of map operations provided by the NModel library.
Maps appear often in models with structured state. Maps may be used to represent
dynamic functions, or key/value relationships that evolve during the run of the
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