levelUp method

void levelUp()

Set parent of current node, as a parent for next operations.

Think about it as: move one up level in stack. If current node is root and this method is called TreeBuilderException is thrown

Implementation

void levelUp() {
  if (_current == _root) {
    throw TreeBuilderException("Already at top of tree");
  }
  _current = _parenthood[_current]!;
}