Among the main relation types between task types, there is
SubTask. This expresses the actual task breakdown for a given
instance. Instances of such relation type are created by the Planner
during task instantiation. Such a relation type may be seen as the
instantiation of the DECOMPOSITION type-level attribute.
Relation types GenInputs and GenOutputs model the horizontal chaining of tasks, i.e. they express partial ordering among task instances. They can be seen as the instantiation of the FORMALS type-level attribute.