1. In a diff the first element may have a slicename, since it need not be the root element as a differential may immediately jump to a subnode by path. If the element is the root, then indeed there may be no slice name 2. is correct. We can male an invariant for (1) If the path has no '.' (a real root), it cannot have a slicename (true for both snapshot and differential) For (2), we need to group by path, and then enforce conditions per-group. I see no way to do that in FhirPath. Alternatively, we would need an axis to get to the "previous" nodes from the current one, having the same path. We don't have that option either. So on first sight, I don't think we can enforce this rule.