code: public static FieldCoordinates systemCoordinates(String fieldName) {
          assertTrue(fieldName.startsWith("__"), "Only __ system fields can be addressed without a parent type");
          assertValidName(fieldName);
          return new FieldCoordinates(null, fieldName);
      }
docs: The exception to the general rule is the system __xxxx Introspection fields which have no parent type and
  are able to be specified on any type
  
  @param fieldName the name of the system field which MUST start with __
  
  @return the coordinates