Remarks
Such a structure can be modeled by a directed tree which we encode by means of a set of NodeAggregationInfo instances.
More precisely, each node of the original graph is mapped to a unique NodeAggregationInfo instance. The mapping is stored in the IMapper<K, V> that is passed to method aggregate. Each NodeAggregationInfo has a reference to its parent which induces a tree structure. There is always exactly one NodeAggregationInfo without a parent that represents the root of the tree. Property nodesOnlyOnLeaves allows to specify whether nodes are only mapped to leaves of the tree structure or if they can also be mapped to inner nodes.
Note that there are NodeAggregationInfo instances that are not associated with a node of the original graph. These instances always represent an inner element of the tree structure.
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