DOM-like xpath to locate WOM object instances (http://www.w3schools.com/xpath/xpath_syntax.asp)
- verb = update, xpath to elements to be updated
- verb = attribute, xpath to elements, the attribute will be updated
- verb = insert, the element will be inserted right before the element specified by xpath
- verb = append, the element will be appended right to the element children elements specified by xpath
- verb = remove, xpath to element to be removed
- verb = removeall, xpath to elements to be removed.