Task trees are often used in the design of interactive software applications and are usually built by human factors specialists [3]. In such models, tasks are represented hierarchically. A well-known notation for task models is ConcurTaskTree (CTT) [4]. CTT includes four kinds of tasks: user tasks, application tasks, interaction tasks and abstract tasks. A CTT abstract task is composed of subtasks connected by means of temporal operators. These operators are: Choice  (T1[]T2), Independent Concurrency (T1|||T2), Concurrency with information Exchange (T1|[]|T2), Order Independence(T1|=|T2),  Deactivation (T1[>T2), Enabling (T1>>T2), Enabling with information passing (T1[]>>T2), Suspend-resume  (T1|>T2), Iteration (T*), Finite Iteration (T(n)), Optional tasks ([T]).

A CTT model is mainly intended to help designers to define interactive applications. However, it has been shown that the same notation can also be used to define test models describing the interaction between the user and the application and providing valuable information about the possible user behavior