We consider the influence of a CAS context on a learner's process of constructing a justification for the bifurcations in a logistic dynamical process. We describe how instrumentation led to cognitive constructions and how the roles of the learner and the CAS intertwine, especially close to the branching and combining of constructing actions. The CAS has a major influence on parallel constructions after branching and it facilitates combining. Hence, the CAS has the upper hand near branching points but the learner has the upper hand near combining points.