This interface represents an XDM Value, that is, a sequence of items.
Note that different implementations of Sequence might have very different
performance characteristics, though all should exhibit the same behaviour.
With some sequences, calling iterate() may trigger evaluation of the logic
that computes the sequence, and calling iterate() again may cause re-evaluation.
Users should avoid assuming that a sequence of length one will always
be represented as an instance of Item. If you are confident that the sequence
will be of length one, call the head() function to get the first item.