This class is meant to be a base class for many kinds of
iterators that require at least one of their children to be
present in the document for a match to happen.
Parses the text in the document.text attribute and fills in the
document.terms and document.tags arrays, then passes that document
to the next processing stage.
A retrieval evaluator object computes a variety of standard
information retrieval metrics commonly used in TREC, including
binary preference (BPREF), geometric mean average precision (GMAP),
mean average precision (MAP), and standard precision and recall.