code: def reset(self):
      
      self._target_by_address = OrderedDict()
      self._target_dependencies_by_address = defaultdict(OrderedSet)
      self._target_dependees_by_address = defaultdict(OrderedSet)
      self._derived_from_by_derivative = {}  # Address -> Address.
      self._derivatives_by_derived_from = defaultdict(list)   # Address -> list of Address.
      self.synthetic_addresses = set()
docs: Clear out the state of the BuildGraph, in particular Target mappings and dependencies.
  
      :API: public