code: public static XField createXField(PreorderVisitor visitor) {
          JavaClass javaClass = visitor.getThisClass();
          Field field = visitor.getField();
          XField f = createXField(javaClass, field);
          return f;
      }
docs: Create an XField object from the field currently being visited by the
  given PreorderVisitor.
  
  @param visitor
  the PreorderVisitor
  @return the XField representing the method currently being visited