Provides standard collection of vistiors. Once the tree is built, the Animator is used to inspect the tree. For each visitor, it walks the tree from source to each receiver (depth-first) and/or from each receiver back up to the source. At each node, the visitor is invoked both prior to accessing it's children (or parent) and afterwards as the recursion backs out of the path. State can be stored at each node, in the links between nodes and cumulative results can be collected and passed back along the visiting path. Different visitors can be combined to perform separate passes on the tree, preparing state on one pass that can be used by subsequent visitors during their analysis.