- NTAs: if we try to evaluate an attribute on a stale copy of an NTA, 
  strange things will happen
- TODO:
  * in fullCopy, use the non-invalidating setParent (same for clone, copy?)
    reset time stamps
  * there's no need to flush caches when adding primitive types
  * in setChild, insert Child, removeChild: flush caches of affected
    subtrees