code: public static ElementSelector byXPath(String xpath, ElementSelector childSelector) {
          return byXPath(xpath, null, childSelector);
      }
docs: Selects two elements as matching if the child elements selected
  via XPath match using the given childSelector.
  
  <p>The xpath expression should yield elements.  Two elements
  match if a DefaultNodeMatcher applied to the selected children
  finds matching pairs for all children.</p>
  
  @param xpath XPath expression applied in the context of the
  elements to chose from that selects the children to compare.
  @param childSelector ElementSelector to apply to the selected children.