This class provides the xml merging apparatus at a defined XPath merge point in
2 xml documents. The MergeHandler that embodies the XPath point can have
embedded XPath merge points, resulting in a cumulative effect with varying
merge behavior for a sector of the documents. For example, it may be desirable
to replace all the child nodes of a given node with all the child nodes from the same
parent node in the patch document, with the exception of a single node. That single
node may instead contribute its contents in a additive fashion (rather than replace).