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]!;
}