Is it possible to specify a when XPath statement that is a negation in a ‘when’ clause? IE, I want a particular leaf node to appear ONLY if another particular leaf does NOT exist. Really I want three fields, A, B, and C. A must always exist. If B exists, then C cannot (and vice versa - if C is instantiated first, then B cannot exist). I know I can enforce this at validation time, just wondering if I can enforce it directly in the yang model as well with a “when” (I couldn’t figure out any sort of negation…I can get existence, but not !existence.