/**
  * Return an iterator over the results of the expression, given an iterator over the principal
  * operand to the expression
  *
  * @param base an interator over the input to the expression
  * @param context dynamic evaluation context
  * @return an iterator over the results of the expression
  * @throws XPathException
  */
 public SequenceIterator getMappingIterator(SequenceIterator base, XPathContext context)
     throws XPathException {
   boolean b = isInstance(base, context);
   return SingletonIterator.makeIterator(BooleanValue.get(b));
 }
 /** Evaluate the expression */
 public Item evaluateItem(XPathContext context) throws XPathException {
   return BooleanValue.get(effectiveBooleanValue(context));
 }