[NTG-context] combining multiple accents

Hello, I always thought that "any accent can be placed on any character" in TeX. However, probably due to some boxes, this fails to work sometimes. So how can I create "a with cedilla and ring above" for example: \r{\c a}? (Yes, I know that I can switch the order: \c{\r a} works as expected, but I need to create more complicated cases in general and I don't want to depend on whether the accent will be placed properly or not.) Is there any simple cure for that? Thanks a lot, Mojca