This interface enables a client to find all nodes in a document that match a particular pattern.
In fact, it allows any subset of nodes in a document to be located. It is used specifically by the
internal implementation of keys. In XSLT, the criterion for including nodes in a key is that they
match an XSLT pattern. Internally, however, keys are used for a wider range of purposes, and the
nodes indexed by the key are defined by a PatternFinder