code: @Nullable
    private String extract(Document doc, String field) throws XPathExpressionException {
      XPathExpression expr = xpathSupplier.get().compile("//" + field);
      NodeList nl = (NodeList) expr.evaluate(doc, XPathConstants.NODESET);
      if (nl.getLength() > 0) {
        return nl.item(0).getTextContent();
      }
      return null;
    }
docs: Runs a wildcard xpath (i.e. "//field") to locate the provided field in the
  document and returns the text content of the first matching node.
  
  @param doc Document to process.
  @param field Field to look for.
  @return Text content of first matching node or null for no match.
  @throws XPathExpressionException If expression is invalid.