Evaluate the expression, returning the result as an IEnumerator (that is,
an enumerator over a sequence of nodes and/or atomic values).
An enumerator over the sequence that represents the results of the expression.
Each object in this sequence will be an instance of XdmItem. Note
that the expression may be evaluated lazily, which means that a successful response
from this method does not imply that the expression has executed successfully: failures
may be reported later while retrieving items from the iterator.