2 Why We Need Model-Based Testing

Systems with Complex State 169
Bag properties and queries
Bags support all of the set operations. The behavior of these operations is similar to
that of sets, except that multiplicity is respected. For example, the Union operation
for a bag adds the multiplicities of each element.
Bags have a few operations that are not available on sets. For example, you can
find out the multiplicity of an element in a bag with the CountItem method. You can
find out the number of distinct elements in the bag using the CountUnique property.
The Count property returns to total number of elements, including multiples.
10.3.6 Pairs and triples
A pair is a data record with two elements. A pair is an immutable value with
structural equality.
Pair nameId = new Pair(1, "Alice");
Assert.AreEqual(nameId.First, 1);
Assert.AreEqual(nameId.Second, "Alice");
The pair constructor takes the paired values as its argument. The properties First
and Second give access to contents of the pair.
A triple is a data record with three elements. Like pairs, triples are immutable
values with structural equality.
Triple nameIdPay =
new Triple(1, "Alice", 2000.0);
Assert.AreEqual(nameId.First, 1);
Assert.AreEqual(nameId.Second, "Alice");
Assert.AreEqual(nameId.Third, 2000.0);
The triple constructor takes three values as its arguments. The properties First,
Second, and Third give access to contents of the data record.
Pairs and triples work well with sets, maps, sequences, and bags. For example,
a set of pairs represents a binary relation. Binary relations occur often in systems
with dynamic relationships between entities.
10.4 Case study: revision control system
In this section we present a case study for a revision control system that uses some
of the data types introduced in the preceding sections.
A revision control system is a distributed software application that helps programmers
work together on a common source base. Each user edits a local copy of
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