DocumentOrderIterator takes as input an iteration of nodes in any order, and
returns as output an iteration of the same nodes in document order, eliminating
any duplicates.
A FixedSortKeyDefinition is a SortKeyDefinition in which all aspects of the
sort key definition (sort order, data type, etc,) are known.
A SortKeyDefinition defines one component of a sort key.
A GroupIterator is an iterator that iterates over a sequence of groups.
The normal methods such as next() and current() always deliver the leading item
of the group.
StandardCollationURIResolver allows a Collation to be created given
a URI starting with "http://saxon.sf.net/collation?" followed by a set of query parameters.