class Tick(object):
  def __init__(self, tree=None, target=None, blackboard=None, debug=None):
    self.tree = tree
    self.target = target
    self.blackboard = blackboard
    self.debug = debug

    self._open_nodes = []
    self._node_count = 0

  def _enter_node(self, node):
    self._node_count += 1
    self._open_nodes.append(node)

  def _open_node(self, node):
    pass

  def _tick_node(self, node):
    pass

  def _close_node(self, node):
    if self._open_nodes:
      self._open_nodes.pop()

  def _exit_node(self, node):
    pass