 of type <code>xdt:yearMonthDuration</code> using the specified
 <code>year</code> and <code>month</code> as defined in
 <a href="http://www.w3.org/TR/xpath-datamodel#dt-yearMonthyDuration">
   XQuery 1.0 and XPath 2.0 Data Model, xdt:yearMonthDuration</a>.</p>
 
 <p>The XML Schema specification states that values can be of an arbitrary size.
 Implementations may chose not to or be incapable of supporting arbitrarily large and/or small values.
 An <CODE>UnsupportedOperationException</CODE> will be thrown wi