Dependency paths of a value model may overlap, as illustrated in
figure 6.1. This models the situation that the Railwaycompany offers group
tickets and two Travelers use this offer.

In general, a value model has one or more customer needs,
each at the root of a dependency graph, and these graphs may or
may not overlap. A trace through the model is a set of
dependency paths, one for each customer need in the model. If
the dependency graphs of the model are disjoint, then we can
partition a trace in disjoint dependency paths and analyze them
separately. But if the dependency graphs overlap, then a trace will
contain partially merged dependency paths that must be analyzed
jointly.