code: public <T> T eval(XMLResource resource, Class<T>aReturnType) throws Exception {
  		T retVal = (T) xPathExpression.evaluate(resource.doc(), getConstant(aReturnType));
  		return retVal;
  	}
docs: Evaluate the XPath on an XMLResource and convert the result into aReturnType.
  
  @param <T> the expected type of the result
  @param resource
  @param aReturnType
  @return
  @throws Exception