Possible enhancements:
[ ] Make adding of an entire subtree smarter by making place for entire subtree at once
    instead of repeat updating the tree for each node iteratively
[ ] String attributes could be used for non-integer
    node names; i.e. string node names would be stored as attributes
    this allows to generalize the use of the library to cases where the nodes are not
    "consecutive positive integer numbers"; numerical IDs could be assigned
    automatically in such cases

