isTable function
Whether handle is a live table of doc: a root-level servable
instance (T1), the rule TableSurvey.of applies. O(1): no survey.
Implementation
bool isTable(DraftDocument doc, Handle handle) => switch (doc.tree[handle]) {
final InstanceNode node =>
node.parent == doc.rootHandle && _isServable(doc, node),
_ => false,
};