code: private int determineVisibilityForSharedFieldAccessor(Field field) {
      if (field.getOwner() == null) { // true field
        int visibility = AstUtil.getVisibility(field.getAst());
        if (visibility == Opcodes.ACC_PRIVATE) visibility = Opcodes.ACC_PROTECTED;
        return visibility;
      } else { // property
        return Opcodes.ACC_PUBLIC;
      }
    }
docs: private to protected to solve http://issues.spockframework.org/detail?id=151