Builds a DOM forest and maintains association from system IDs to DOM trees.
A forest is a transitive reflexive closure of referenced documents. IOW, if a document is in a forest, all the documents referenced from it is in a forest, too. To support this semantics, {@link DOMForest}uses {@link InternalizationLogic} to find referenced documents.
Some documents are marked as "root"s, meaning those documents were put into a forest explicitly, not because it is referenced from another document. (However, a root document can be referenced from other documents, too.)
@author Kohsuke Kawaguchi (kohsuke.kawaguchi@sun.com)